The (red) window will reveal the next number when you have advanced the right amount. After you develop the film you can examine the backing paper and see the numbers are staggered to line up behind red windows in different places on different cameras. (Cameras that have inserts to do 6 x 4.5 may have two windows, you advance first to the one window and then to the next window.)
I always wondered how 645 people measure the distance since they don't display that numbering...
Mamiya: 7 II, RZ67 Pro II / Canon: 1V, AE-1, 5DmkII / Kodak: No 1 Pocket Autographic, No 1A Pocket Autographic | Sent w/ iPhone using Tapatalk
~Stone | "...of course, that's just my opinion. I could be wrong."~Dennis Miller