How to replace bulbs in this ceiling fan?
I almost  certain that the glass should rotate off, the little indent in the metal will be for a  corresponding groove in the  glass cover. Some times the friction on these make them feel as thought they will not move.
If you are careful and wrap the glass in a damp towel, or some other grippy material, and try turning with a little more force it should come free. It should be counterclockwise to undo it.