With parts like the rare sights from S&S, there could be a small mark added on the bottom of the base.  If I made them, I would so mark them.

But that is far from the only part or practice that may well cross the line.  Take a look at the practices of those who "restore" Winchesters from the pre-Krag and Krag era.  It is possible to buy newly-made roll stamps that mimic the markings (models, makers, patent numbers) on the barrels of the originals.  You can have worn serial numbers "touched up" or new but "old looking" numbers engraved. 

You can buy new stamps with inspector's marks and cartouches.  Seems to me that the guys who make & sell that stuff do so for the benefit of those who deliberately cross the line.