Steer clear of the 545/545i backs. These backs were for sleeved, single sheet, instant film. This film has not be available new since 2008, and nobody else makes any instant film for these backs.

The 545/545i backs are easily identified by a toggle lever on the face of the device. (see link)

Also, remember that peel-apart instant film cannot be frozen for long-term storage, (the cellophane tube of developing chemicals rupture when frozen, and will then harden and dry out).

http://benneh.net/archives/lf/polaroid.jpg