I have several of these. They don't need a sheath; the spring holds the film in the right place. I think they're intended to be dual-purpose film/plate holders; the spring has enough give to accommodate the thickness of a plate as well.

I've never had a problem with them, but I remember seeing someone say that they found theirs didn't quite sit square in certain cameras. I've used them on Voigtlaender, Nagel, and Kodak AG/Nagel cameras successfully, though.