That adapter is redundant on a Spottie, though. They have a bridge type meter -- that is, the meter needle is measuring the *difference* in current through the metering cells and the speed dial potentiometer; higher or lower supply voltage will result in more or less needle displacement with incorrect exposure, but the center null will be correct (at least if you don't have a completely dead battery or so much voltage you fry something from the excess current).

I've used my Spottie with silver oxide cells, with about 25% higher voltage than the original mercury cells, for a couple years at least, and the exposures are fine.