The nearest I can remember recently was the infamous idiot on Freeads seemingly mating a BSA Buccaneer barrel to a Scorpion and claiming it was a rare BSA prototype. Along with all his other bogus attempts to describe his recent auction purchases as long-standing items from his personal collection that had been serviced by his “airsmith”.

There was a thread recently on Collectables about an auction of “Theoben prototypes”, which description was probably a bit misleading.

And, though I have zero evidence for this, it wouldn’t surprise me if at some point someone had in good faith, say, knocked up a replica BSA military pattern that later got represented as an original.

Plenty of questionable “Venoms” around, of course.

But I’ve never heard of anything like the thing you mention. Is that an actual thing, or a thought experiment?