Thanks to you both. I had to put it away for the time being yesterday and plan to get back at it tomorrow. Luckily it's for my own apartment versus for a resident, so I have a bit more time to fool with it. One thing I also didn't try was to loosen the 3 bolts. I figure that might loosen it a bit, enabling me to move the tabs from the ring.
If that doesn't work, I'll start trying your suggestions. Never had one like this. I suppose it's time for a learning opportunity.
Tgoods, this may sound like a stupid question, but where in the flange do you place the allen wrench?