See http://lommen9.home.xs4all.nl/holders/index.html for clarification about the different back types.

Note the comment under '120 Roll Film Holder' - "These newer roll holders are made for film which is thinner than the older film. The new Rapid Vance Roll Holder have extra guide rollers to hold this thinner film pressed against the film gate." I ran into this problem about 1960 and the company came out with a spring modification kit to hold the (then) newer films flat. There may still be unmodified backs around.