If a fake is made up by a knowledgable individual, with the intent to deceive, it will be harder to spot than one assembled as a representative specimen. My 1903A3and3/4 is a case in point. It looks like an A4, but would not pass inspection by anyone with any knowledge of what a genuine A4 is like.
The price of some snipers has been rising astronomically, and the incentive to produce fakes is there. Careful study of reference material, like the entries in the MKL, and textual references is really important. Often items are sufficiently uncommon that it might be impossible to have actually handled a specimen prior to purchase. For example, I know a collector with Mk.III Ross/Winchester A5 sniper, so I could actually examine one.
From time to time someone on this site will proudly post a photo of a recent acquisition, and the PMs start to fly. "Did he really think that he was buying a genuine.....? Should someone tell him....?"