Oh. Sorry about the confusion!

Two sizes of Fuji Pack film
  1. FP100x is 4.25x3.25 inches, and can be used in the Polaroid 405 back, the Fuji PA-145 back, and the Polaroid 'Automatic' series of cameras (that's another topic, best served by looking for The Land List web site
  2. FP100x45 is 4x5 inches and can be used in the Polaroid 550 back, the Fuji PA-45 back, or some Polaroid passport cameras


In both cases x can be B or C (B&W or Colour), and actually even the number can change (being the film speed: i.e. FP100; FP400; FP3000; are 100, 400, 3000 ASA respectively).

The Fuji numbering system is pretty consistent actually! The only fly in the ointment is that there is sometimes a choice of finish - gloss or 'silk', but that's stated in the film name. If 'silk' isn't mentioned, then it's gloss!)