Well, in theory using the voltage tester might help, but it won't necessarily tell you what bulbs you have in there. If you measure 24 volts on the drivers side wires, you might assume you have
24 volt bulbs, but there a high likelyhood you'd be wrong.
MC9s came with 12v bulbs installed in series on a 24v feed. However, in the years preceeding the MC9, the bulbs were indeed 24v bulbs, and they transitioned to 12v bulbs in the very early MC9s. Yours is one of the very early MC9s if it is 1979, and therefore, it's entirely possible your bus came with 24v bulbs in it. The only way to know for certain is to trace the wiring and see how it's
wired, or look at the existing bulbs to see what they are (assuming someone didn't already make the mistake of putting 24v bulbs in place of the 12v bulbs; which would burn dimly
I would have probably been wrong then because I would have bought bulbs for whatever voltage I got on the volt meter. Of course I would have tested the volts at the actual socket where it plugs into each bulb. This 12V in series is good info to learn..especially since I'm buying an MC-9 tomorrow. Are MC-9 stock marker & clearence lights also 24V?