By "threading" I mean inserting the edges of the film into the folds. It can be done by starting both sides at one end - or inserting one edge completely first, then bending the film slightly to go under the other fold. I find the second method best.
There is no slot on UK and Continental book holders. The plates were loosely placed down in the frame and then held in place by a combination of 3 latches and the central metal plate with springs. Sometimes there is no spring when the latches match the exact thickness of the glass plates.
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