I recently purchased a kit advertised as a "late xm16e1". After going through the xm16e1 identification thread, this seems to check out. Whether or not the parts are matching is something I should have asked, but they all seem to date to 1966/7 which would be accurate to "late xm16e1".

Anyhow, there's a couple markings I can't figure out. On the back of the carry handle are the numbers 11  and 153, I assume this is something unrelated to original factory markings, but I was curious if someone knew what it was. Also the along with "MP   12" on the barrel, it is also marked "P" on the opposite side under the FSB. I saw some posts on here referencing what the "P" could be but they didn't all seem to agree on what it was. Maybe I've missed something obvious in one of the posts on this forum, but was hoping someone might know what either of these markings are.

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Rack number.

That’s a rack number, I have one on mine as well. The MP 12 barrel dates it to roughly 1966 not sure about the extra P stamped on the barrel

I have an original XM16E1 upper that had a rack number at some point that some rumdum sanded off and tried to re-finish.

I just built the rest of the gun to go ahead and look like it had seen some shit

Ah I see, interesting. Is there anything to glean from these numbers like where the guns may have to been throughout their existence?

No not that I’m aware of

Nope, a rack number is a rack number. Every infantry company in the Army had about 100 M16s assigned to it and they were all numbered 1-100 for every company.

What is interesting to me though is that in 26 years in the Army, 1984-2010, I never saw a rack number stamped into the metal of any weapon.

I saw them painted on, Dymo labelled, safety wired dog tags (mainly on spare barrels for crew served weapons) but never stamped into the metal.

The Army gets quite pissy when you permanently alter weapons systems.

Same here, during roughly the same time period. The stamping could have been done by a LE agency or foreign military, but I've never seen Army weapons stamped like that.

Very nice nonetheless.

U.S. Army from 1978 to 2001, I never saw a rack # stamped in either  As you said, someone doing that would get their a$$ in a sling for altering rifles!
I used to get in trouble just by taking the FCG out of them when I was cleaning it before inspections! How else are you going to get all that carbon and dirt out?
My guess is foreign military as well