It may just be an old wives tale, but if you look at gun for sale ads and some dealer catalogs going back almost 100 yrs., you will see the xxx effect when the serial number was mentioned and the explanation I heard was always that a crook could claim the gun was stolen from them, etc., resulting in problems for the current possessor in proving legal ownership. You had th same effect with surplus military firearms that legal owners removed the US stamp because it was known that law enforcers would confiscate US marked weapons they encountered on the theory they were stolen ( as some were, of course ). In the days past that I operated a gun store, more than once the sheriff showed up with a warrant to seize a gun I had purchased, or taken in trade, as it was listed on his stolen gun list. The store's video showed the seller and at least once the seller was the person who reported the gun stolen. Of course there weren't any xxxs on the paper work and sometimes the sheriff's warrant would have a matching serial number but not the correct make or type of the gun, so I didn't surrender the gun. Being a law abiding citizen, I recorded all purchases and sales with their full serial number, however I had the thought of adding a number with xxxs to .22 rifles I sold ( no serial number was required on them for many years ) just to see who was awake at the state DOJ and ATF were the records were kept! Ed.