New old Poseidon score

[IslandKraut]

That was my first plan of attack. Once removed, the cover at the back exposes a small diaphragm and what appears to be a switch that pressure activates the bottom timer.

I managed to get the face loose enough to peek behind it, and it appears as if the battery is mounted directly onto a circuit board attached to the back of the dial face.. It's just a matter of working out if its possible to get to it without destroying the mechanism or the case.

Yes, I know the bottom timer is essentially pointless when I have a computer on each wrist. And I never look at my SPG because one of those computers has AI.. But still, it's nice to get things working as they were intended...

The face o-ring doesn't look too difficult to install properly again.. It fits nicely into groove for a face seal.

I wouldn't touch those two screws as I suspect they calibrate the needle.
May also not be needed at all. Dacor had gauges like that, and they were powered off a 1/2N battery inside.
The hatch on the back BTW is NOT the way in but rather holds the diaphragm to cover the switch that activates the button.

To get to the inside:
1) unscrew the bottom fitting. It should have a few little orings where it seals against the plastic and inside mechanism
2) take off front cover
3) wiggle the whole inside dial assembly forward and out the front, but mind the little wires that connect to the back and battery holder. You may need to push the assembly up a little through the bottom where the fitting was as you try to coax it out the case
4) light swearing at this point is optional but likely
5) once out, unscrew two little phillips screws that hold the battery cover on
6) replace with 1/2N battery or two LR44 stacked and isolated with shrinkwrap
7) put the batt cover back on
8) gently insert the dial back in (sometimes useful to push the bottom fitting forward to allow the back end to slide in (back rubber end of pencil is great for this)
9) wiggle it so confirm proper seating
10) put on the oring and clear plastic lense cover.

The unit can be tested by pressing the button on the back cover gently (don't poke the diaphragm)

Good luck...