In a recent large acquisition, I got a bunch of film reels. I think they are Jobo reels for the 1500 tanks. They are white (or were, once upon a time) plastic, much like the Paterson reels only much thinner and with no ball bearings. On the outside of one side of the reel is a red tab. When pushed in place, it sticks into the film path. What is this for? Is it supposed to allow two rolls of film on one reel? And when would that be useful?