I differentiate my Mustangs by color ("the silver one" or "the white one") or age ("the older one" or "the newer one"). Same with AR-15s - the spray painted one, the shiny one, etc.
This AR-15, a Spike's midlength, I now refer to as "the burned one." It remains serviceable, reliable, and accurate, with the only parts replacement being a new top MOE handguard - the bottom remains the same. It's photographed here with the old handguard for...dramatic effect?
Whenever I get some free time (probably sometime in 2014), I will do a few more things to it, then clean it and do some diagnostics/examinations.