would you mind if someone tagged your car with spray paint? the car would still work fine.
If it's an "S", the guy might have been doing what I always do. In Basic Training we were told to use a laundry mark for everything we were sending to Quartermaster Laundry Service which was to consist of the initial of our last names and the last four digits of our service numbers. I engrave mine under the right grip panel on my pistols. Maybe that's what this is.