A WW2 German Mystery: Knorr Bremse FG42 Prototype?

This is really a mystery rifle, or half of a mystery rifle. It is part of the collection at Springfield Armory National Historic Site, but its exact provenance from World War Two is not recorded. It is catalogued as a Knorr Bremse product, which is plausible but there is no specific marking on the rifle to confirm that. Photos have existed online for a while, and it is widely regarded as an FG42 prototype because of its short length and side-mounted 7.92x57mm size magazine. However, Knorr Bremse was not ons of the companies that participated in the FG-42 program. So is it perhaps a late-war concept to replace the FG? Or something else entirely? We just really don't know, but I thought it was worth filming the gun to show the elements that are not visible in the online photos.

Mauser train video:
https://youtu.be/qjkjfOhNSx0

Thanks to the Springfield Armory National Historic Site for giving me access to this truly unique specimen from their reference collection to film for you! Don't miss the chance to visit the museum there if you have a day free in Springfield, Massachusetts:
https://www.nps.gov/spar/index.htm

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