What's your reason for thinking the power cylinders only have one valve. It looks like an F-head engine, with one valve in the block, and one in the head. I think some Jeeps or Land Rovers had this type of engine. The other four cylinders could be air pumps. Maybe that's when the gauge is for, monitoring the air pressure. Or maybe it's an early air-gasoline hybrid. And why is the intake so small, and not tapered? Where's the intake air filter or pickup? Or possibly, it's a gasoline start, diesel run engine, like the foghorn air compressor at East Brother Light in San Francisco Bay. I'm wondering if both engine share a crankshaft, with an overrunning clutch, or perhaps the levers at the front are a clutch mechanism to disengage the diesel from the gas starting engine.

What about a 2cycle aircraft engine with two large scavenging blower cylinders? I would be willing to bet no one had yet invented vane type superchargers or turbochargers at the time the photo was taken. It looks like a WWI (or possibly earlier) aircraft engine prototype to me. I know they were experimenting with 2cycle injected gas engines trying to achieve better performance in the thin air at higher altitudes.