Ah right the C is not the same as the ETRS series.
It is possable to shoot 120 in the 220 insert. You'll have to start the roll at a different point n not at the arrow.
Also the frames will get very close as you get closer to the end of the roll. I have done this many times with my RB67s.
What i sugest is experiment with a roll of paper backing. With the lens off.... Advance the film to find your frame 1 start point.... It should be almost at the very tongue end.
Then once you establish a start point.... advance the frames, mark them with a pencil so you can see how they align. Just be mindful of hiw many frames you shoot.