There are 3 wires attached to it and the wires feed into the wiring harness. Yes, that's the voltage regulator to the right and below. The choke heater control is the small module that's bolted to the bolt at the heat well and it has 2 wries, one that goes to the heater coil and the other goes to the coil terminal. Other than the choke heater, the engine has none of the other features you'd expect to find on the <6100 GVW engines such as the heat stove on the exhaust manifold and the flapper in the air cleaner snorkel, no EGR, air pump or anything. My '77 D150 and '74 D300 have none of that stuff, including no choke heater. My folks old '73 D100 did have the choke heater and heat stove/flapper, but no air pump. I may have to take a multimeter to the wiring harness to see where the 3 wires go.
Fortunately I took photos of the parts truck as I disassembled it. I'd like to know what the square module located in the upper left corner of the photo is, the one fastened to the inderside of the fierwall. This '77 W100 truck had the supplemental electric heater on the choke. Neither of my other trucks have the heater, and neither do they have the module. However, when describing the heater, my factory shop manual only shows and mentions the small ceramic control module mounted right next to the choke well. Anyone know what this is? Thanks!