Any suggestions??
I suspect it will be a trivial exercise for someone who knows how they work
Well, it may only be trivial if it hasn't all become rust-bound. That notch in the disc does look like it's intended to be used with a drift and hammer, to unscrew a left hand thread.
The cross-holes
could be intended to take a rod to lock some part of the assembly, while the drift is used.
You may need to use a small inspection mirror, to see which inner bit would be held stationary by an inserted rod, here.
And a good soak with a
quality penetrating lubricant [Penetrene, CRC 5-56, Inox etc.] is an essential preliminary step.
