It's not a solenoid, per se. It's called a deceleration valve. One port goes to manifold vacuum, one port goes to the distributor vacuum advance can, and one port goes to the advance nipple on the carburetor. What it does is when the throttle abruptly closes and the manifold vacuum increases above the valve's setpoint, it admits full manifold vacuum to the distributor advance can for emission reduction. Virtually every marque in the late 60's early 70's used a variation of it at some time. This is the AMC version.