For anyone collecting/using old cameras you will eventually encounter the battery replacement issue. Starting in 1991/92 there was a restriction on the sale of mercury content in batteries … this lead to the end of all mercury (mercuric-oxide) batteries. Silver-oxide batteries became the best replacement to powering cameras, but this battery used a higher voltage than the 1.35V mercury batteries, which means the older camera meters have to be recalibrated/modified to use them.
WeinCell makes replacements that are of the equivalent voltage … but they ain’t cheap, so here are some alternatives”
The O-ring and Jon Goodman’s 675 adapter are described by Rick Oleson: Cheap Easy Mercury Battery Replacement
same topic by Bruce Varner, here: http://brucevarner.com/PX625Replacement.html
… and here is another discussion: Mercury-Oxyde Battery Problem