Indestro, Patented also, 2,065,340, may be on the bottom in very tiny type. The hex bar should also be stamped Indestro or such (duro?) unless someone lost it and replaced it with an allen wrench ;P
But..
They seem to come in red and green .....
The green ones perhaps are unmarked for contract production ???
I have seen a lot more green ones than red ones, but have only seen the red ones marked Indestro....hmm