Original Leitz Cassettes (FILCA) & the FED Version
This shows the reloadable cassettes made for the cameras.
On the left is an original Leitz one and in the middle is the one that replaced it in 1931 (Code FILCA) and, on the right, is a FED version. It's a copy of the Contax one, by the way. (In other words, in the former USSR they had the best of both worlds with a copy Leica body that took Contax style cassettes* and - with the Jupiter 8 lens fitted - had a copy of the Contax Sonnar(?) f/2 lens.)
The Leica ones have a spring clip holding them shut and the little knob on the top (at the side of the knurled centre spindle) will turn once the cassette is in the camera and the base plate is fitted. Turning the lock on the base plate to the closed position opens the cassette and closes it at the end of the film.
For the FED version, the taller of the two little knobs goes down and frees the inner to rotate, etc. And the slots, being open one end made it easier to load.
By the way, modern cassettes are shorter than these old (meaning pre-war) ones and their copies. So a felt or cardboard spacer washer is needed to align the film correctly. Put it between the camera base plate and the cassette or, perhaps better, use two; one above and one below the cassette: I've yet to find the time to sort this out. If you don't you get the exposure over the edge of the perforations and a black edge opposite when prints are made. The black line will usually be above the sky in a landscape shot. The film may also be twisted slightly...
* I have since discovered, by looking carefully at my cameras and the cassette, that the FED one only works in the Kievs and the FED 2. It might work in the Contax but my old Contax II is away being repaired.