The bottom of the 22525300 (GT172) factory cap found on many 78-up B, D, C, Y, and G-body gas cars looks like this belly up. I think early versions had a handle, but I could be wrong. Seems like I recall that vaguely for some reason. Anyway, it appears there is some sort of valve assembly inside the cap. So there's at least one valve involved. It appears there's some sort of spring-like apparatus inside, leading me to believe the way it's setup is to vent into the tank to prevent vacuum. Of course, I could be wrong. I've never taken one apart or cut into it to determine what's going on in there. It's one of those things I never thought about because I've always run the charcoal canister. So I knew it was always vented, regardless.
The way it's made with the vent holes and everything make me lean to some sort of vent path, but then again, it depends on internal construction. They may just use the same mold for the insides and simply plug the guts. Who knows?
Not sure if I have an old, dead cap laying around somewhere or not, or I'd cut into that bad boy and see what was up. Maybe use a hole saw on the top cap cover and peek inside.