I'll hazard a guess that it's CMT.
Crappy picture, but it sure looks like a DPMS rifle freefloat tube in the picture to me.
I don't know any other companies offering the exact same tube with the ball milled grooves, but there are certainly lots of other basic freefloat tubes out there as well.
They would certainly be made on a contract basis so if you need one, it's going to come from DPMS.
CMT would not be able to sell you a product they manufacture for someone else.