Tiny numbers and marks, hand-scratched on the inside of the back of a case, are most likely watch repairer's or jeweler's marks. When watches were serviced, the watch repairer would place his mark and/or a date code inside the back. The marking might include a code to indicate the type of service that was done. Many times, the codes were intentionally obscure so that so that they'd be less likely to be faked. Other times, the marks may be the watch repairer's job numbers. Thus, when the watch came back, the watchmaker would check the marks and instantly know just when he (or she) last worked on it.
Originally Posted by Jon Hanson