That is some real dated stuff there.
The throttle positioner was a vacuum dashpot that raised the curb idle slightly higher when egr was in use, which was a real bad idea at low engine speeds, i mean its feeding exhaust gas into the intake, engine does not like it. It leans it out without adding oxygen and in turn reduces combustion temps and the production of Oxides of Nitrogen.
Sooo, the vacuum amplifier used the weak vacuum from a port that had what was called venturi vacuum that raised as the engine speed increased, vs regular manifold vacuum that goes down. Why ported vacuum like was hooked to the distributor advance wasn't good enough i don't know. They didn't want this to run on a cold engine because driveability would be horrible (stalling, hesitating, popping back through carb) so they added the ported temp switch in the thermostat housing to only work above 170 F maybe.
This is not stuff found on most trucks, they got a pass for most of this stuff until late 70s pretty much what griff says
