I'm a Turkish student studying in Armenia and while we were crossing the land border with my Israeli friend, no one else but us got these abbreviations (I guess) written on our passport stamps (and we were a large group of people with many different nationalities). Knowing that the relationship between Turkey & Israel, and Armenia is not super nice, I assumed it may have something to do with that. :)
All Bagratashen – Sadakhlo land border. On the top, enter into Armenia, bottom left leaving Armenia, bottom right entering Armenia again (we had to leave-enter-leave Armenia in an hour due to luggage problems).