I was under the impression watchmakers often use/used their own markings and trying to decipher them years later was likely impossible
This. As far as I know, each watchmaker had their own shorthand so they could tell if they had done the service before, when, and what was done. Impossible to interpret. Thank God this has all but disappeared except for a few really, really old watchmakers who refuse to give up on it. I hate having the case back all marked up, even if it is hidden on the interior.
In some places a national organization would assign watchmakers identification marks that they would scratch inside your case back, but as noted this is considered damaging a customer’s property now, so not done much anymore.
The marks could be anything from initials of the watchmaker, to a date, to an internal job number...