https://ogrforum.ogaugerr.com/...03#76748553614826303
https://ogrforum.ogaugerr.com/...7#152477234593134587
That's broken pieces of the Lexan plastic case rattling around the inside of your transformer.
It's likely a clutch drive screw that was used to assemble the case. It was not meant to be easily disassembled.
If you have a sentimental attachment to the transformer, I'd leave it as-is.
TRW