Originally posted by Brooke Meyer The only difference is the length of the roll. 220 is longer because its thinner without backing, more can fit on the spool. There is no way to differentiate thickness. The tab position is simply an indicator of how long and therefore how many frames are on the roll.
Put a roll of 220 in a 120 back and the pictures will be fine except the camera wiil rewind half way through the roll.
I would disagree with that. There are two differences between 120 and 220, in fact you say them yourself :-
1) The film is twice as long.
2) The film has no backing paper other than the leader and tail. (Of course, it is this that allows (1).)
As for rewinding, I don't know the 645N, but no 120/220 camera I have ever handled had a rewind : the film just winds onwards to the take-up reel where it stays until processing.
There is a different pressure plate requirement between a 120 back and a 220 back. The gate in front of the film against which it is pressed stays the same, but a 220 pressure plate must be slightly further forward by the thickness of the paper that 120 film would have. This might not sound a lot, but it is critical to holding the film flat without excessive friction. On fixed back cameras like the Pentax 6x7 this is done by the user shifting the plate sideways slightly to place it onto a different step. For 220 film, the back will also of course need to be capable of counting to a higher number, but the 6x7 body can cater for the higher count anyway.