There are no markings whatsoever on the upper or barrel designating caliber.
The barrel is a heavy barrel profile, 20" long and devoid of any markings. I've taken the hand guard/forearm off and found that to be the case beneath the hand guard too.
There is a square that seems to have been cast into the upper just above the forward assist. I'll attach a photo.
That is the only marking on the whole assembly.
