The law requires guns imported into the USA to be stamped with its caliber and the company that imported it. According to Rock Island Auction, collectors do not like these import marks and stamped guns sell for a lot less than non-stamped guns. Take the Lugers in the above photo. The stamped Luger sold for 60% less than the unstamped Luger despite both being in similar condition.
If you buy a gun to shoot, this is not an issue, but if someone tries to sell you a Luger make sure you look for an import stamp.