The earliest they stopped production of the 125 would have been 1978 some time as I have a high arch 125cc model that came with the LM carburettor which was introduced 1978. Also it had the black "press once only" label on the deck pointing toward the primer cap, not pointing away from it where a remote primer may have been fitted.
I had a neighbour with the exact same mower decades ago, 125cc and all and specifically recall the same label and it's position with LM white primer cap.
I think they more likely stopped selling 125cc models even later as 1980 was when they stopped selling Mayfair style low arch bases as the bottom end model and offered thumb latch alloy high arch mowers in lower price point models.
BTW I believe the 125s with an LM or G4 carburetor is a hidden gem as the ones I've used have decent power over the G3 I used to have which was a bit lacking in the tough stuff. May have been faulty but I doubt it. Just like a bigger carburetor was used in a car to give more performance, I think the plastic carburetor has that effect on the 125 and maybe even a 160, you just don't notice the difference as much on the bigger engine.
