The MGA Coupe door lock originally was fitted to the left side door only from the first Coupe (20671 Sept 1956) up to car number 25109 (January 1957). For the British design, that would be the kerb side (passenger side for RHD car) when you drive and park on the left side of the roadway. This makes sense (sort of) for a single design, as most of the cars were built for export with left hand drive. The key lock would be on the driver side for the majority of the market which was North American export. The key lock was operated by the ignition key. The inside lock was set by lifting the inside handle. Both doors would unlock with a push on the inside door handle, even if the key lock was locked on the outside (and it would remain locked on the outside).

That single key lock (for four months of production) does make for some interesting inconveniences. For the RHD market the driver must use the key to unlock the kerb side door, open, enter, reach across to unlock the far side, exit, close, then run around to the off side to enter and drive. To park and lock up the driver may lift the inside handle and close to lock the off side door, then run around to the kerb side and use the key to lock the kerb side door. Not too bad for the gentleman when entertaining a lady friend. Just let the lady in AFTER reaching across to unlock the far side. The rest seems natural (at least to the lady).

For the LHD market the driver can use the key to unlock the street side door, then enter and drive. At end of trip he would exit, close, and use the key to lock the door (leaving the passenger door locked). So far so good, until the gentleman needs to let his lady friend into the car. Duh? "Please wait on the curb, sweetie". Then run around to the street side, use key to unlock, enter, reach across to unlock the far side, exit, run back to the curb side, open the door to let the lady in, close, run back around to the street side to enter and drive. At end of trip the driver would exit, lock the street side door with the key, run around to the curb side, open to let the lady out (hoping she hasn't locked it from the inside), lift the inside handle to lock, and close the door (to stay locked). You can imagine how may of these cars were regularly left unlocked.

From car number 25110 on (January 1957) the single key lock was fitted to the driver side door. This then requires two different part numbers for the door assembly on each side of the car (depending on which side has the key lock). Service Parts Lists for all MGA models (1500, 1600, Twin Cam, after the change) list the same four part numbers for Coupe doors, and always a single key lock. The difference is that early Coupe had the lock always fitted to left side door, and later cars had the single lock fitted to the driver side door. The prototype coupe which is shown in "Original MGA", by Anders Ditlev Clausager, has the key on the right side, but that is presumably peculiar to the prototype car.

At the same production point (car number 25110) there was a change of the inside locking door handle. See Confidential Service Memorandum MG/202. You can lift the inside handle to set the inside lock and close the door. If you do that for both doors you will not be able to get back into the car, not even if you have the key in your hand. You may wish to disable the inside locking function for the driver side door (or the door that has the single key lock) to prevent locking yourself out of the car. More about that on the following page.

Addendum:
I believe there may be an error in CSM MG/202 (link above). For earlier production with single key lock on left door only, the single inside locking handle must have been on the right side door (which would be the only way to lock that door). The CSM indicates it was previously on the left-hand side only, which must be wrong. After the change point both sides have the inside locking handle (which leads to the problem on the next page).