When did they have the serial on a separate piece of metal riveted to the BB? This BB has the holes in the right place for the rivets, so maybe it fell off?

Some municipalities back then would stamp a serial in a bike that didn't have one, a requirement to get the bike license sticker. OP's serial sure looks like it was stamped later; they even had to put some touch-up paint over it, almost certainly because the frame would have rusted from being stamped after being painted originally.