This car came with a 12 Volt Bosch electrical system
A 12 volt system is much more robust for battery life, starter cranking power, generator efficiency, and headlight brightness. So why was this car equipped with a 12 volt system but most other cars did not switch from 6v to 12v until the 50s or 60s? It turns out that the reason 6 volt was the standard for so many years was because of reliability problems of the head lights. The filament on a 12 volt bulb is much more delicate when lit than the big husky filament of a 6 volt bulb. In 1915 streets were either a dirt road or cobblestone. Asphalt or concrete surfaces were rare so most cars bounced around quite a bit while driving. If you ever see old film clips of a Ford Model T you will see that it looks like a mechanical bull trying to toss it's rider. This harsh bouncing would blow out a headlight bulb pretty quickly, making auto owners want to return to the acetyline gas headlights from 1912 even though they were not very bright and were difficult to operate.
Bosch announced that they had cured the problem by making the filament of their 12 volt bulbs more durable. The brochure that you can browse on this page is a result of this breakthrough.
Offering 25 candlepower no-maintenance headlights along with the other benefits of 12 volts must have seemed like a easy decision for Marmon engineers.
Unfortunately Bosch was a little optimistic. Most cars STILL blew bulbs at an unacceptable rate. My Grandfather found that this particular Marmon did fine because the 36 inch tires and heavy weight of the car smooths out any surface (I find that driving off road is just as silky smooth as asphalt) AND switching to Mazda brand bulbs allowed bulb replacements to happen every few years instead of every few weeks or days. The generator on this car has never needed service or adjustment as far as I know.
Because of Bosch jumping the gun, most cars were sold with 6 volt systems for decades. Even Marmon switched their cars to 6 volt in 1916. I have seen at least one Marmon that had an 18 volt system. It used 12 volt to start the car but used 6 volt to operate the lighting.