The camera's light meter was originally powered by a 1.35 volt PX400 mercury cell. Now that Mercury batteries are now banned in most countries "Zinc-Air" cells can now be used instead. Personally, I use a "Wein" Cell Part No. MRB400 1.35V Zinc/Air Battery for my Pentax Spotmatic II. This is replacement for the now obsolete RM400R and V400PX battery. The 1.35V Zinc/air cells are available locally in Singapore and are relatively inexpensive.
This is the preferred solution; otherwise if you want to use a 1.55V Part No. 387S Silver-Oxide cell you would need to modify the meter circuitry.