Electronic Spark Control, as well as EST, Electronic Spark Timing.
Not sure which is which, but I suspect ESC are the distributors that have vac and mech. advance but use a knock sensor to retard timing.
There are EST setups out there, and those distributors have no vac or mech advance. The EST computer controls all aspects of timing, and would have a knock sensor as well.
CA trucks could have CCC, which is the computer controlled carb, and of course a distributor that is completely controlled by the ECM.