Hi Joel, do you mean the film speed indicator? None of the Mamiya TLRs have light meters, so changing the number in the display cut out on the C3 doesn't have any affect on the film's exposure, it's just a visual reminder of what film you have loaded. I made a little window with a magnet to attach a 120 box top to my C3 despite that display.
Odd that the newer, improved C220 doesn't provide that indicator, but perhaps they thought showing whether the film was 120 or 220 was enough.