The older model Russian PSO scopes were designed to operate on a battery that is now obsolete.  The answer is this adapter that fits in the battery well/compartment of the original PSO scopes.  These are needed to convert a PSO scope to 3 volts using D-357 batteries. 

Recommended for some POSP models as well with the original PSO battery well design.  Will work in RPG / PGO optics.  Just keep in mind this converts the optic to 3 volts and you will need to have a 3 volt bulb to match the new power source.

If your battery well is about 23 mm in diameter, this is the battery adapter you have been looking for.