I do not know much about how they are produced, or how they work, really. Something about magnetic strips against the film, I think. I am probably wrong. Where did you find this? There might be a player or recorder somewhere on the same shelf, or at least in the same vicinity.

[Without any labels, though, it was quite likely they were blank. She peeks onto the shelves to check for herself, looking around the items arranged for a cassette player.]