Reduce silver oxide battery 1.5v to 1.3v
My vintage Spaceview runs fine directly on the silver oxide battery, but I know that not all do. 



I think a small Schottky diode could be added internally so that non-mercury batteries could be used thenceforth. Given the costs for servicing these finicky watches, you could probably charge $100 or more to add a 10-cent diode such as this one. 



There are even smaller diodes that might work too, but if you stick to a fairly low voltage rating and a fairly high current rating the voltage drop should be around 200mV at low current.