Wow, a new working PX625!
When that dies, you can use an MR-9 adapter that takes a 386 1.55V silver oxide battery and adapts it both physically and the output voltage, to reduce 1.55V to the needed constant 1.35V.
Beware!...On eBay, there are now phony 'MR-9' that do not do any voltage conversion, but rely upon 1.35V Zinc Air batteries that deplete themselves as soon as you peel off the sticker, even if the camera is unused and turned off! Carefully read the text description to make sure you are reading about MR-9 that does
voltage conversion, before you place any order
Buy the MR-9 while the PX625 is still functioning, to double check that the MR-9 convertor is doing the proper job and resulting in 'same reading' as with PX-625. I have an original MR-9 from US seller CRIS, purchased literally decades ago, and thought I would have a second one in hand, so I ordered this recently., at a considerable savings from CRIS current price....
https://www.ebay.com/itm/130646083632
Nice looking Leicaflex, BTW!