That's part of the TRC (Throttle Return Control) system that's described on page 6E-26 of the 1979 service manual. Thanks to
hatzie you can download a copy of that manual -->
http://67-72chevytrucks.com/vboard/s...d.php?t=545416
Description from the manual:
The TRC system used on heavy duty emissions vehicles consists of three major components:
(1) Throttle Lever Actuator - Mounted as part of the carburetor assembly, this device opens the primary throttle blades a preset amount in excess of curb idle when engine vacuum is applied to it. This actuating vacuum is controlled by a separate solenoid control valve.
(2) Solenoid Vacuum Control Valve - Mounted separately from the carburetor, this off-on valve is held open above a present nominal engine speed by a signal from an electronic speed sensor. The valve when open allows a vacuum signal to be applied to the throttle lever actuator as long as the preset engine speed is exceeded.
(3) Electronic Speed Sensor - Mounted separately from the solenoid vacuum control valve, this switching device monitors engine speed at the distributor and supplies a continuous electrical signal to the solenoid vacuum control valve as long as the preset engine speed is exceeded.