The A386 uses the El Primero 3019 PHC high-beat movement, with 36,000 vibrations per hour (5 hz) permitting measurements to the tenth of a second, which at time of release (1969) compared to the competing Caliber 11 at 19,800 and Seiko 6139 at 21,600, and the standard 18,000 vph (2.5 hz, measuring to a fifth of a second).
Higher frequency brings greater precision, with fewer variations in rate due to external factors. Zenith compensated for the watch losing power more rapidly due to its high beat by incorporating a mainspring with 50-hour power reserve.
The movement may contains additional letters stamped on the plate of some pieces, which were related to production logistics of the time, and were likely a method to keep track of different batches. To date, I haven’t been able to discern a pattern. Letters may appear both above ‘SWISS MADE’ on the plate (A, B or C), and then also just to the left as (A, B, C or M; exact location varies); the letters do not need to match, though they may. If a letter appears above ‘SWISS MADE’, it does not mean that a letter will necessarily appear to its left. Likewise if there is a letter to the left, it does not mean there will be a letter above ‘SWISS MADE’, though it is quite rare that one would not (far more rare than a letter appearing above ‘SWISS MADE’, yet not to its left).
There are two different rotors that were used across the different production batches.
In both Mk 1 (538Dxxx-539Dxxx) and Mk 2 (706Dxxx-708Dxxx) we see the most common rotor, with two lines of text, reading ‘ZENITH’ on the top line, and then ’31 JEWELS’ and ‘SWISS MADE’ on the second line.
Once we reach Mk 3 (861Dxxx-862Dxxx, 922Dxxx-923Dxxx, 230Exxx-233Exxx) we start to see increasing use of different rotor stamping with each of the batches. The late rotor stamping is seen rarely on the first batch of the Mk 3 (very late pieces only), but on a reasonable percentage (around 1/4) of the second batch, and then more of (around 1/2) of the last batch.
 Zenith Heritage department, email May 11, 2020