If you are no using the Leica re-loadable film canisters, place a small cone shaped spring on the exposed bottom of the film cassette, before replacing the bottom plate, or you may very well see exposed sprockets because, the pre-M cameras holders are longer than a typical, modern 35mm film canister, which can drop down in use.
Also, when cutting the film leader, BE SURE to not start your cut by cutting through the sprocket hole, but between them, otherwise, little bits of broken sprocket can and will accumulate in your camera.
Leica had a brass, folding leader temple that allowed for a fast trim with a sharp pocket knife, and a close second choice, if you can no find an original user, was being made but I do no know where to find one at the moment.
I loved my IIIC and hope to own another Leica Barnack or M-film user one day, but always keep it in mind that you do no need the trimmer ( I used a pair of folding scissors or my Swiss Knife) or a card to load these cameras and a little practice will really increase your ease and speed, at film changing.
P.S. use the spring and remember it's there when you go to open the camera!
IMO.