And, to all, on this subject of Polaroid backs, remember that the Polaroid backs that have a large toggle lever on the back, (facing the photographer when installed), these backs are the ones labeled 545 or 545i, are [absolutely and totally] useless for instant photography today.
Also, I don't have any kind words for those individuals selling 545/545i backs today, without first informing others to the uselessness of these backs. And no, I have not bought one of these backs, however I do keep a list of those individuals that sell these backs. It is becoming a fairly long list. It's my, "Crooks!...Do Not Buy From!" list.
Fuji doesn't make instant film for these backs, and the Impossible Project doesn't seem to be considering making any instant film for these backs.
The Polaroid backs for use with three (3) of Fuji's instant pack films, "FP100b" (B&W 100 ASA), "FP100c" (Color 100ASA), and "FP3000b" (B&W 3200ASA)
are called a "405" Back. These films are 3.25 X 4.25 inches in size, 10 sheets per pack.