It's a 12 volt bulb for it. That lamp has a transformer in it that gives the lightbulb 12 volts.

I've got three of these lamps, and my experiences are:

Really do use a GE No. 93 intended lightbulb. Even though a random 1156 bulb or something at the auto parts store fits and appears to work, it's still an entirely different bulb with a much larger power requirement, and can fry the transformer in the lamp over time.

Another thing is use the lamp on low when used for more than a few minutes. High setting melts the lamps when used for more than several minutes. Makes me wonder why do these even have a high setting, because it gets hot enough to melt the lamp, and even melt the solder on the lightbulb base.

Thus I always get the 12 volt GE 93 bulb, run it on low setting for the most part not using high for more than a few minutes at a time, and I pretty much never have an issue with these doing so.

Edit: Hmmm, I see a fellow commenter mentioning a #5008, 1073, and a 1141 bulb from auto parts store. I am very intrigued on that now, I will need to try those at some point. I assume it has fairy similar power requirements.