The first HD3 had a Detroit 2-71 and it's rare. The 2nd HD3 was based on the D15 farm tractor. Came standard with a shuttle shift coupled to a gear transmission. The two-speed "Power Director" was optional. Has dry steering clutches that can be removed without taking off the final drives - similar to a Deere 450. Diesel engine is direct injected - NO glow plugs. Just an intake-manifold "air heater." Original steering clutch discs were fiber and optional or new ones are metallic.
HD4 is quite different. Radiator is in back by the dashboard, has a bigger 200 c.i. engine, and torque-converter drive. Undercarriage is the same as the HD3.