I just spent ages trying to find the meaning of the secondary markings on Glasgow imported watch cases.
In the past a cursory glance showed what would appear to be a pointer to the company supplied with the case so where A1M was stamped I assumed that it was either A Michel or something like Marvin. I hadn't noticed that the number alters from case to case.
If anyone sees a mark beyond 12 let me know, because if no one has already said it I am hazarding a guess that it stands for Assay Month with the number of the month in between, as the position on the cases and the stamp seem to show that it probably wasn't done in Switzerland.
Thus A1M should be January, A12M December, if it stands for the month of the year. This doesn't help with the theory that assay letters went from halfway through the year, but may indicate that this is meant to simplify the situation in layman's terms.
So let us know if you have a 13 onwards, then it could mean Assay Master.