Reloadable cassettes; from the left an early Leica version (FILCA), the later one called the 1B, a modern one, a Leica M Series one and, finally, a Contax one.
The main difference is the height of the container. For Leicas with the screw thread lenses the taller ones are needed or else a (felt) spacer in the gap at the bottom of the camera. The older ones are a fraction of a millimetre under 47mm across the container (ignoring the spool end). Modern ones are about 42½mm, hence the need for a spacer. Best done as two spacers, one at the top and one at the bottom, imo.
The Contax ones are easiest to fill in the dark and there's a lot of clones made in the old USSR that are both cheap and plentiful.