Returning the GUID of an unknown CustomXMLPart in PowerPoint's CustomerData object

I'm experimenting with saving custom XML data to [shape] objects in PowerPoint using the object's CustomerData collection object which stores one or more CustomXML parts. There are three main methods and one property for the CustomerData object:

.Add (method)
.Item (method)
.Delete (method)
.Count (property)

Unlike other collections in the OM, items in the collection are read using a GUID string rather than a numerical index. for example:

Sub TestReadXML()
  Dim GUID As String
  GUID = "{CBAC284D-9000-4CFE-8E8F-FAD5CC8BCAA7}"
  With ActiveWindow.Selection.ShapeRange(1).CustomerData
    With .Item(GUID)
      Debug.Print .XML
    End With
  End With
End Sub

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The GUID is created automatically by the .Add method when the CusomXMLPart is initially created so at that point it can be read and saved:

Sub TestWriteXML()
  Dim GUID As String
  With ActiveWindow.Selection.ShapeRange(1).CustomerData
    With .Add
      GUID = .Id
      .LoadXML "<testXML/>"
    End With
  End With
End Sub

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Now, given the fact that the GUID cannot be present in more than one CustomXMLPart, either for the same object or multiple objects, when an object that contains a CustomXMLPart is copied, the original maintains its GUID but the copy is assigned a new one automatically.

Let's say I have a shape selected in the example above and I add the XML part, then copy/paste the shape, the new shape has been assigned the same XML data but with a different GUID.

How can I find out what that new GUID is?!

Is it deliberately inaccessible for security reasons or is this just an incomplete part of the PowerPoint OM?