[morzh]

Shadowmeters will usually have 4 wires: two are the galvanometer (vane movement) and two are the lamp.
In case one of the lamp's connections is through the chassis (clip-on) there may be 3 wires.

For instance in 18 the lamp is not grounded so there are two wires to it.
I do not have 37-630, but in 37-640 the lamp is grounded so it is possible it may use only one wire, but may also use two, depending on what kind of lamp socket it is.

This is it. It is a milli-ammeter with a light bulb.
Looking at it again, I am pretty sure this wire goes to that spare terminal. That is, I am sure. Without pretty.