In 71' GM cali cars had a system to disable the vacuum advance when in certain gears. Referred to as "spark controls" it used a switch to ground the circuit to a solinoid that would either allow or block vacuum to the vacuum advance can on the distributer.
If I remember correctly it is a discontinued part. Might be found used. Same switch used by all GM from 71-75 vehicles.
Note: it is a 1/8" npt port which happens to be a common plug size.
