Hi Harold,
Its the lightblue part,
As I cant get Alibre working at the moment due to the lisenceserver not granting my request, I am not sure of the name.
The extra Part was buried three (3) layers deep in a subassembly. I went down the trees until I found it and suppressed it. The "corrected" model is attached.
Mareike,
If you edit sub assembly Mat'54 Bollekop and Hide, Delete or Suppress part BK326 Roof Basic 21327
then the extra part will be removed with this result:
I'll let you edit your model rather than uploading another model to take up space on your computer.