I've seen posts from people saying they bought a FW that has had the serial number filed off and the assumption was that at some point it was stolen. How many times can something be bought and sold before it's no longer "hot"?? I think I would feel funny buying a machine if I knew its serial number had been mutilated. Was it stolen back in the 40's, or was it stolen 40 minutes ago?
I guess that explains why we sometimes come across machines where someone has scratched their SSN into the paint. A lot of these vintage machines went for the equivalent of thousands of dollars when new - makes sense they'd have been something worth stealing.