It came to me where i saw one of those little round things, not exactly the same but similar
it was in a 1950's marquette battery charger, it also had selenium rectifiers, and it had a breaker that looked alot like that round unit, it just had heavier posts on it, maybe 1/4" or so.
i got that charger given to me because it didn't work, got it working and had it around for years, all my buddies called it "R2D2" because of its resemblance to the star wars robot.
always hearing "hey somebody know where R2D2 is?" around the shop...
i gave it to a buddy and i suspect ol' R2 is still being drug around to charge up someone's hotrod.
bob g