Here is an auction that features some of these, with a description of the type in your picture:
https://www.robertedwardauctions.com...rd-collection/
While I can see the potential for someone wanting to fake a proof from 1990 Donruss (especially the examples of proofs that exist for cards that didn't make it into the final set) I would have some doubts someone would go to the effort to fake a Keith Miller card... Then again, stranger things have happened!