This shows (on the left) the two cylinders of the Leitz reloadable cassette and (on the right) the version of the Contax one made in the former USSR. As you can see, the Contax version has open slots and the film on the spool is much more easy to insert.
With the little button depressed the FED one is the same height as a Fuji, Ilford or Kodak loaded cassette. That means 47mm but the 30's and so on Leitz and Carl Zeis (or Leica and Contax) reloadable cassettes are 49 or more mm high.
This ex-USSR one was made by FED. It has two little bits sticking out at the end and these engage with the base plate lock. The longer one goes down as the base plate is fitted and frees the inner cylinder, enabling it to rotate open. That is why on, f'instance, FED 2 there are two locking tabs to be rotated shut. The one under the cassette opens and closes it as it is turned.
NB The FED 2 takes both FED reloadable cassettes and norml commercial ones that come with film in them.