Adamsguns, the guy that got raided by the feds a few months ago, has a very good webpage about the NFA weapon system that ATF runs. He says 1,200,000 guns in the system. After reading the article, I wonder if anyone knows the number.
When I worked for a big Class 3 Dealer back in 1973 and 1974, I asked how ATF kept records of the transfers. Supposedly, my boss had been to Washington and seen the system that was in use at the time. According to him, each gun had a index card showing every transfer and the more transfers, the more full cards were attached to the last entry. Kept in cabinets similiar to a Dewey Decimal System in the library.
There was always a fear that a registered gun could become illegal if the cards somehow got misfiled or jumbled. If the card was not found the form was denied. So everyone gathered as much paper on every gun so they could prove registration if needed. The only time a card was touched, was when a request for that certain gun was made.