Stuck motometer radiator cap

[Nlbloch]

I have a Ford script motometer with the temperature gauge showing through a small window. The upper half of the cap seems to be stuck to the lower half, such that the entire motometer housing comes off the radiator when I try to open it. I’ve removed it and tried to open, cleaned the rust, tried WD-40 to lubricate the turn — no luck. I do not get any movement when trying to compress the two halves, which I know is needed to “unlock” and allow the turn.

Any suggestions?