What do these AR15 upper markings indicate?

[roydamnmercer]

These are three basic PSA uppers, all the same “freedom week” type special. Two are the same type free float and one is a Magpul Moe standard. I cannot see any difference in the receivers…in fact, they look as if they could have been produced at the same time. One is a year old (the Moe) and the other twins are brand new.

So what is the meaning behind the letters on the rear takedown pin hole stud?

Any thoughts CTS Tactical ?