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Hi mattknapp

120 film is indexed by advancing the film until the next frame number comes into view. The 14mm red disk gets glued into a recess inside the camera for viewing frame numbers. These days, I print disks from a red translucent PLA, but a clear red presentation folder (think Office Max) works too. The red is a traditional color for this little viewing pane. The camera is designed such that stray light from the window probably shouldn't affect your film, and the pane is probably not strictly necessary. I cover the window with a bit of gaffer's tape on the back of the camera, as I often leave a given camera loaded with film for a while and I want to avoid fogging my exposures. HTH! Todd