Did you try turning the entire metal vent, not just the glass? It looks like maybe if you give it a quarter turn you will undo the latches.

I suspect there are a couple of "tension clips" on the outer rings to push down and "release" the cover.

Yes, tried twisting the whole fixture and I thought it looked like clips too, but it’s just fixed metal. The clip looking bits are firm, rather than tension. 1980 fixture fail!

If you have any idea of the manufacturer, they probably have an installation guide on their site or a customer service dept... Also, the quarter turn idea has merit, "quarter" being the operative word. In what room is this fixture located? If a bathroom, perhaps steam and humidity have affected the metal clips?

Bathroom. I don’t see any manufacturer info on the fixture. I wouldn’t be surprised if 40 years of steam has affected the clips. To me it looks like it should be a quarter turn that listens but it won’t budge a bit. Maybe the universe is telling me to just get a new fixture!