Mercury Replacement Batteries for Pentax and Olympus Cameras
The light metering in the Pentax Spotmatic cameras and some older Olympus cameras were originally powered by 1.35V mercury batteries; the MRB400 and MRB625.
Mercury batteries are no longer produced and alkaline or silver oxide batteries give too high a voltage.
The Wein Cell mercury replacement battery is a zinc-air battery that gives 1.35V and is recommended for use in the Pentax Spotmatic and Olympus OM-1 cameras.