Whoa!!! I was just in high school when the 67' standards were in service. However, if my memory serves me correctly, the MDCs worked this way: The "Close End Door" would close the associated zone and the end set of side doors of the conductors car. The Center door in the conductor's car remained open so the conductor could insure that all other doors were closed and no one was trapped. Then the "Close Center Door" button would be pressed and the center set of doors in the Conductor's car would close and the train would proceed. Does anyone remember it differently?
BTW, one of the afore mentioned cars is being rebuilt in Coney Islnd by volunteers. Photos to follow!