For automatic transmission equipped cars, it is the "idle return" dashpot.
It has an internal diaphragm, plunger and external spring around the plunger.
When the throttle pedal is released, the throttle snaps shut, and the dashpot slows that last bit of closure of the throttle plates to avoid stalling.
As for the spring, any coil that fits between the trunnion and the bolt head. But you may not need the spring.
When you set the trunnion turnbuckle correctly, it is minus one turn after contact with all the throttle plates closed.
The bolt is turned in to have both secondaries all in at WOT.
The normal return springs should facilitate the proper movement.
Test it and see. I am not sure if the spring was an attempt to fix the "binding" issue that some people had.
The correct part number for the spring is: 1373763
But I do not know where you can get one.