Yours seems to be a bit different from the one I recently removed from the '78 Sovereign
, but here is what I did.
The inner circle should unscrew (if it's made the same) from the tank throat. After unscrewing the inner rotatiing element (the toilet gasket seat) the outer area of the flange should lift off.
To start the inner seat rotating I used a small chisel and a small hammer, and GENTLY tapped (rotated) it out.
Even if you mess up the entire flange I believe the hold down assembly is still available.
More info in this thread: