Every factory DPMS barrel I've seen has the caliber and twist stamped on it between the gas block and muzzle. The upper and float tube look right, the gas block and barrel maybe not. Hard to tell from the pic. Looks like a muzzle device maybe? I don't know of any bull barrels that came that way. Not for sure, but I don't think so.
Is there a sling stud? When replacing barrels on rifles with non-indexable float tubes it's a real pain to get the sling stud back on the bottom afterwards. But maybe the barrel was pulled, fluted and threaded and put back on. See if there is some caliber stamping between the flutes at all.