AFAIK they fit _nothing_ else :-( The ribbon cartridge is a plastic box about 3" * 2.5". It fits on top of the right sideframe in the Model 43, and it's held down by a magnet on the chasiss and a metal plate on the bottom of the cartridge. There's a long loop of ribon that runs from the cartridge round rollers on the Model 43. There are 2 on the left sidefram and 2 on the carriage, and IIRC they're fitted with one-way brakes, so that they turn one way and lock if you try to turn them the other way. As the carriage moves left-right, the ribbon doesn't move (rollers on the carriage free-wheel, ones on the fram lock), as the carriage moves right-left, the ribbon is pulled along with the carriage (rollers on the carriage lock, those on the frame free-wheel). That is the _only_ ribbon feed mechanism, there's no drive to the cartridge or anything. I've never seen any other printer that works like this. -tony