The MGA Coupe door lock was fitted to the left side door only up to car number 25109. For the British design, that would be the kerb side when you drive and park on the left side of the roadway. This makes sense for a single design, as most of the cars were built for export with left hand drive.

From car number 25110 on the lock was fitted to the drivers side door. See Confidential Service Memorandum MG/202. It is not clear if this meant both sides or the drivers side only (whichever side the driver occupied). This would mean the door skin had to be selectively punched depending on LHD or RHD, which would then make four different part numbers for the doors. The prototype coupe which is shown in "Original MGA", by Anders Ditlev Clausager, has it on the right side, but that is presumably peculiar to the prototype car.

The door lock was operated by the ignition key. The door without the lock could be locked from the inside with reverse motion of the door latch handle. Both doors would unlock with a pull on the inside door handle. It is not clear if the non-key-lock door would originally lock with a motion on the inside door latch handle (probably yes), but it could certainly be a retrofit part regardless. There is an article in "MGA!" magazine from NAMGAR about disabling the interior locking function on the door with the key, to prevent accidental lockouts, after which you must use the key to lock the door.