IIRC, the question of other (hidden) serial numbers has gone to court, the defendant admitting grinding off the butt number, but claiming that since there were numbers he didn't grind off, the gun still had a number and he was not guilty of the charge. He was found guilty, the prosecutor's argument being that his intent was to cover up a crime; that he missed some numbers didn't make him innocent of defacing the number he knew about.
Jim
Jim