Yes it is specific to the 6pk and hard to find , been NS! for a very long time , if you find one it may not be cheap .

It is used to set engine idle RPM. Chrysler had a run on issue, I can only assume, so the easiest/cheapest way to fix it is to have the butterflies closed completely when you shut off the engine. The solenoid is powered up when you turn on the key and it open the butterflies, it is what you are supposed to adjust to set idle speed.

And no your 4bbl doesn't have it , though some 4bbl's had them , but they are different than what is used on a 6pk.