David, older 23 backs have a carrier with knob advance and a shell that doesn't have a pin roller at each side of the gate. Newer ones have a carrier with lever advance and a shell with a pin roller at each side of the gate. The newer shells, with pin rollers, are supposed to hold the film flatter than the older ones, without. Note that carriers interchange between the two types of shell, so it is possible to get a lever-wind carrier in a shell without pin rollers and a knob wind shell in a carrier with.
Also note that to accomodate the pin rollers, the new type shells' gates are about 4 mm narrower than the old type.
I have a new type 23, it has other problems but holds the film flat enough.