It's possible early in the Vietnam war. The Armalite AR15 is what we now know as the M16. They had select fire AR15's from Armalite before they sold the rights to Colt and it became the M16. But it's not the same rifle that we call an AR-15.
I think you are right. Just could be a case of conflating the two to confuse the issue. I never saw an Army weapon that had AR-15 marked on it.
