How did my watchmaker know what year my watch was serviced?

To be fair, there's a difference between a company inspector making marks in a railroad watch (at the time, an essential safety tool that was paramount in the safe operation of the railroad) and a watchmaker making marks on a privately owned watch that does not carry nearly as much importance in relation to the safety of other people.

As a bridge inspector, I make marks on bridges all the time to track defects and mark areas that need to be repaired. I'd be pretty insulted to be grouped in with someone painting graffiti just for the hell of it.