To add some more details to this thread for historic purposes, I wanted
to let you know how I built a tool to remove the ring holding the glass
to the front.

I found some scrap aluminum that was as thin as the slot in the ring.
A steel ruler would work also but the aluminum is softer and less
likely to mar the face of the vario.

I sandwiched the thin aluminum between two pieces of pine board,
holding it together with small C clamps. Allow the aluminum to stick
out from between the boards by no more than 1mm (1/16"), or 1/2 the
depth of the slot. This combination gives some mass to the "tool" and
the wood won't mar the face of the vario.

Take the tool in one hand, the vario in the other (over something soft
just in case it drops), engage the slots, and turn the tool CCW.

Worked like a champ.