The front distributor 1200 engine was the first of the A12's This was followed by the revised engine in 1974 which is also called an A12, so to avoid confusion, if you have an original front distributor 1200 engine, then I suggest that you call it a "1200" engine.

The A12A was basically a late A12 engine that was offered in 1979 to '82 US model 210 cars [B310 chassis code] These were essentially A12 engines but with a 75mm bore that yeilded 1237cc.
They couldn't call it an A12 as that was in use & they couldn't call it an A13 either as it was too small & that code was also in use anyway.

A12-1/2 just didn't cut it, so they went with the A12A identification. [Both A's are capitol letters on the actual engine]