Since there's no commercial label, they're almost certainly bulk loading cassettes, as AnselMortensen already stated.
That being said, if they feel light, they're probably just empty cassettes. It would be easy to check, but you need someone who's familiar with bulk load cassettes, so they could check in a darkroom, or with a changing bag. Don't do this yourself if you've never done this before, since you risk exposing any film, if there is film in there.