OK, I did the test - and the X socket is dead. So for me to have gotten a flash before, I must have plugged into the M socket. Which works. But obviously not synched.

I unscrewed the X socket and it came out with broken wire. Either the unscrewing broke the wire, or a broken wire explains why X did not trigger the flash. Anyone have a step by step set of instructions for taking the top of the camera off? I'd love to re-solder the wire and see what happens.