How do I remove this ? Speedmaster III/IV/125

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Good evening all
I’m trying to switch out the yellowed L-Shaped gasket that holds the crystal in my Speedy 125.
There is a metal inner ring with a 45deg chamfer on it that I guess I’m going to need to remove as the foot of the L gasket sits beneath it.
I cannot ascertain whether this is a screw fit or push fit.
Can anyone offer any direction here ?
Thanks in advance.
Garp