Problem with rear driver side cetral lock [LHD]

I have noticed that from time to time the lock on the driver's side rear door does not lock the door after pressing the button on the remote control. Sometimes you have to press the button several times and open and close the door. I am afraid that it is at the end of its life and will stop closing or opening after some time.

I checked and I have a 5ta839015j lock, later replaced with a 5ta839015m. Both models are hard to find on the market in my country as used [very few offers] and/or at crazy prices.

In addition to above numbers, is there another 5ta839015 rear left lock with a different letter at the end that fits the SIII?