What the original cell was
PX625, PX13 and MR9 name the same 1.35V mercury oxide meter-cell family used in vintage cameras and light meters (Olympus OM-1 is a common example). Mercury cells are obsolete and should not be sought as new stock.
Zinc-air path (Wein MRB625)
Wein MRB625 and similar zinc-air cells target the 1.35V class and the PX625 form factor. They are a chemistry substitute, not identical mercury behaviour — follow the device caveats and expect finite air-exposure life once the tab is pulled.
Adapter + silver oxide
MR-9 style adapters let you use a smaller silver-oxide cell (often 386 / SR43W) to approximate the original size and voltage. The adapter is required — a bare SR43 is not a drop-in for a PX625 chamber.
Alkaline risk
Alkaline “PX625” / LR9 lookalikes are often 1.5V with a different discharge curve. They can fit physically and still throw the meter off. Prefer a documented 1.35V-class path for meter accuracy.
Do not confuse this network with 6V 4LR44 / PX28 cells used in Canon AE-1 class bodies.