Mystery new part: possibly a Steib side car spare tire hub cover.

[Grant R26]

Hey folks !
This interesting round thing-y, came to me with a large collection of NOS Butler BMW parts; many still in original Butler bags.

Included was an original Steib sidecar shock which recently found a good home.

I assume this round plate might also a Steib part and hope somebody out there can kindly identify it for me?

My theory is that it is meant to fit over the spare wheel to keep the road dust out of the brake drum / spline cavity.

See the photos where I've fitted it to my R51 wheels. It seems to be a good fit though it would need to be packed with a rag or gasket to make a good dust-tight seal.

Any input on what it might be or who i can ask about it would be greatly appreciated.

Grant in Toronto