Tamiya's opt port is 7.2v and if I recall correctly, the Russian light harness had resistors in it, so I'd imagine they are straight battery voltage as well.

I wire all mine with LEDs, whether it's a Notek, Bosch or convoy and just go with 7.2 on the calculations.

I assume you're talking the bulbs like on the M4, best bet is to throw a multi tester on it.

I use the lights on most of my Tamiyas as an indication that the switch is on (opt port), but if you're looking to go through the board so you can turn them on/off (or brake lights on the Russians), you can just assume a max of 8.4 volts (fully charged battery) and go from there.