Does anyone here know if an Astra 900 bolt will work in an Astra 903 upper assembly.
One in a million chance I know.

It may....If the 903 uses a sear trip like a 902 (which is activated by the return of the bbl extension / upper)

Perez has a good breakdown on the 902 / 903 / Mod. F differences in his book..... but it's in Spanish

Leonardo M. Antaris (in Davenport Iowa) wrote the big book on Astras .....a quick google search turns up his contact info.....