Concentric cassettes are two brass tubes about 1" diameter with a section of the side removed. Each one has an end closed and they fit neatly into each other (one being slightly smaller in diameter.)
The bobbin with the film goes inside and you leave a tail of film out thru the gap and then rotate the one on the other until it locks shut. The film inside is now in a tube closed to light with the bit of film sticking out.
The camera has a mechanism built into the baseplate or backplate that locks it onto the camera then rotates the tubes until the slits are aligned and the film is free to run. No felt to be seen.

As you open the camera to retrieve the film the mechanism closes the cassette first before unlocking the baseplate.
Unfortunately every brand has its own slightly different design and even then there is variation between models. You have to try before you buy. They last for years. Mine have - on the shelf.
Edit - just took delivery of 150m (450' +) of Polypan F for 65 Euro incl post. Less than a buck a roll.