Fra våre venner i USA, NRA:
The "faking" of firearms is not a new phenomenon. Back in the 1800's the practice of peddling shoddy merchandise marked so as to fool the unwary into thinking he was purchasing a quality gun was not unusual. Witness the many 19th century single shot percussion pocket pistols marked "Derringer" or "Deringe" or some other variation of the famous Henry Deringer name, or the European copies of S&W Model 3 Topbreak revolvers that even went so far as to duplicate the S&W barrel address markings.
Faking of collectable firearms with the specific intention to defraud collectors probably first became a significant problem in the early post WWII years, beginning most notably in the Colt field. As the dollar premium paid for high condition specimens and rare variations increased, the monetary incentive to make and deal in faked merchandise has increased, and spread into many fields.
Historically attributed guns - Remember, paper is generally easier to forge than steel. Beware of faked documents. It can be a very good idea to confirm factory letters - forgeries are known to exist. Pay attention to what the documents actually say (see also "Pieces of History" chapter in the 16th & 17th editions of the Blue Book of Gun Values). Another approach has been the restamping of serial numbers on guns to match those of known historical guns.