Oh. Sorry about the confusion!
Two sizes of Fuji Pack film
- 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
- 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!)