diff --git a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
index 6e86818b1d2370..e9b7912ce26caf 100644
--- a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
+++ b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
@@ -877,10 +877,6 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\BStrStringMarshaller.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\ContiguousCollectionMarshallerAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\CustomMarshallerAttribute.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\CustomTypeMarshallerAttribute.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\CustomTypeMarshallerDirection.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\CustomTypeMarshallerFeatures.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\CustomTypeMarshallerKind.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\MarshalMode.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\MarshalUsingAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshalling\NativeMarshallingAttribute.cs" />
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/MarshalUsingAttribute.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/MarshalUsingAttribute.cs
index f10c2743a60210..fefeba869b1503 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/MarshalUsingAttribute.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/MarshalUsingAttribute.cs
@@ -10,7 +10,7 @@ namespace System.Runtime.InteropServices.Marshalling
     /// This attribute is recognized by the runtime-provided source generators for source-generated interop scenarios.
     /// It is not used by the interop marshalling system at runtime.
     /// <seealso cref="LibraryImportAttribute"/>
-    /// <seealso cref="CustomTypeMarshallerAttribute" />
+    /// <seealso cref="CustomMarshallerAttribute" />
     /// </remarks>
     [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.ReturnValue, AllowMultiple = true)]
     public sealed class MarshalUsingAttribute : Attribute
@@ -26,7 +26,7 @@ public MarshalUsingAttribute()
         /// <summary>
         /// Create a <see cref="MarshalUsingAttribute" /> that provides a native marshalling type and optionally size information.
         /// </summary>
-        /// <param name="nativeType">The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomTypeMarshallerAttribute" /></param>
+        /// <param name="nativeType">The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomMarshallerAttribute" /></param>
         public MarshalUsingAttribute(Type nativeType)
             : this()
         {
@@ -34,7 +34,7 @@ public MarshalUsingAttribute(Type nativeType)
         }
 
         /// <summary>
-        /// The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomTypeMarshallerAttribute" />
+        /// The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomMarshallerAttribute" />
         /// </summary>
         public Type? NativeType { get; }
 
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/NativeMarshallingAttribute.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/NativeMarshallingAttribute.cs
index c4883cba97a46b..2c0d604b191965 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/NativeMarshallingAttribute.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/NativeMarshallingAttribute.cs
@@ -10,7 +10,7 @@ namespace System.Runtime.InteropServices.Marshalling
     /// This attribute is recognized by the runtime-provided source generators for source-generated interop scenarios.
     /// It is not used by the interop marshalling system at runtime.
     /// <seealso cref="LibraryImportAttribute"/>
-    /// <seealso cref="CustomTypeMarshallerAttribute" />
+    /// <seealso cref="CustomMarshallerAttribute" />
     /// </remarks>
     [AttributeUsage(AttributeTargets.Struct | AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Delegate)]
     public sealed class NativeMarshallingAttribute : Attribute
@@ -18,14 +18,14 @@ public sealed class NativeMarshallingAttribute : Attribute
         /// <summary>
         /// Create a <see cref="NativeMarshallingAttribute" /> that provides a native marshalling type.
         /// </summary>
-        /// <param name="nativeType">The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomTypeMarshallerAttribute" /></param>
+        /// <param name="nativeType">The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomMarshallerAttribute" /></param>
         public NativeMarshallingAttribute(Type nativeType)
         {
             NativeType = nativeType;
         }
 
         /// <summary>
-        /// The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomTypeMarshallerAttribute" />
+        /// The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomMarshallerAttribute" />
         /// </summary>
         public Type NativeType { get; }
     }
diff --git a/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Microsoft.Interop.SourceGeneration.csproj b/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Microsoft.Interop.SourceGeneration.csproj
index 02621feb273367..46e36c4dd721b7 100644
--- a/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Microsoft.Interop.SourceGeneration.csproj
+++ b/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Microsoft.Interop.SourceGeneration.csproj
@@ -9,12 +9,12 @@
 
   <!-- Code included from System.Runtime.InteropServices -->
   <ItemGroup>
-    <Compile Include="$(CoreLibSharedDir)\System\Runtime\InteropServices\Marshalling\CustomTypeMarshallerKind.cs"
-             Link="Production\CustomTypeMarshallerKind.cs" />
-    <Compile Include="$(CoreLibSharedDir)\System\Runtime\InteropServices\Marshalling\CustomTypeMarshallerDirection.cs"
-             Link="Production\CustomTypeMarshallerDirection.cs" />
-    <Compile Include="$(CoreLibSharedDir)\System\Runtime\InteropServices\Marshalling\CustomTypeMarshallerFeatures.cs"
-             Link="Production\CustomTypeMarshallerFeatures.cs" />
+    <Compile Include="..\..\tests\Ancillary.Interop\CustomTypeMarshallerKind.cs"
+             Link="V1\CustomTypeMarshallerKind.cs" />
+    <Compile Include="..\..\tests\Ancillary.Interop\CustomTypeMarshallerDirection.cs"
+             Link="V1\CustomTypeMarshallerDirection.cs" />
+    <Compile Include="..\..\tests\Ancillary.Interop\CustomTypeMarshallerFeatures.cs"
+             Link="V1\CustomTypeMarshallerFeatures.cs" />
     <Compile Include="$(CoreLibSharedDir)System\Runtime\InteropServices\StringMarshalling.cs"
              Link="Production\StringMarshalling.cs" />
     <Compile Include="$(CoreLibSharedDir)\System\Runtime\InteropServices\Marshalling\MarshalMode.cs"
diff --git a/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs b/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
index 940490d75dc28b..c5fa43d32d698f 100644
--- a/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
+++ b/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
@@ -2217,42 +2217,6 @@ public struct GenericPlaceholder
         {
         }
     }
-
-    [System.AttributeUsageAttribute(System.AttributeTargets.Struct)]
-    public sealed partial class CustomTypeMarshallerAttribute : System.Attribute
-    {
-        public CustomTypeMarshallerAttribute(System.Type managedType, System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerKind marshallerKind = System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerKind.Value) { }
-        public System.Type ManagedType { get { throw null; } }
-        public System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerKind MarshallerKind { get { throw null; } }
-        public int BufferSize { get { throw null; } set { } }
-        public System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerDirection Direction { get { throw null; } set { } }
-        public System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerFeatures Features { get { throw null; } set { } }
-        public struct GenericPlaceholder
-        {
-        }
-    }
-    [System.FlagsAttribute]
-    public enum CustomTypeMarshallerDirection
-    {
-        [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
-        None = 0,
-        In = 0x1,
-        Out = 0x2,
-        Ref = In | Out,
-    }
-    [System.FlagsAttribute]
-    public enum CustomTypeMarshallerFeatures
-    {
-        None = 0,
-        UnmanagedResources = 0x1,
-        CallerAllocatedBuffer = 0x2,
-        TwoStageMarshalling = 0x4
-    }
-    public enum CustomTypeMarshallerKind
-    {
-        Value,
-        LinearCollection
-    }
     public enum MarshalMode
     {
         Default = 0,
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/CustomTypeMarshallerAttribute.cs b/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/CustomTypeMarshallerAttribute.cs
similarity index 100%
rename from src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/CustomTypeMarshallerAttribute.cs
rename to src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/CustomTypeMarshallerAttribute.cs
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/CustomTypeMarshallerDirection.cs b/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/CustomTypeMarshallerDirection.cs
similarity index 100%
rename from src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/CustomTypeMarshallerDirection.cs
rename to src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/CustomTypeMarshallerDirection.cs
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/CustomTypeMarshallerFeatures.cs b/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/CustomTypeMarshallerFeatures.cs
similarity index 100%
rename from src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/CustomTypeMarshallerFeatures.cs
rename to src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/CustomTypeMarshallerFeatures.cs
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/CustomTypeMarshallerKind.cs b/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/CustomTypeMarshallerKind.cs
similarity index 100%
rename from src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/CustomTypeMarshallerKind.cs
rename to src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/CustomTypeMarshallerKind.cs