Vintage Double Hose

I have a couple of old steel cylinders that I am unsure of the tank volume. They are 6"x20.5", the pressure is 2015psi, the manufacture is PST. I am thinking about adding them as another set of old doubles unless I can find a third cylinder and a triple manifold. I know what size they aren't.

That sounds like they are possibly military surplus oxygen or co2 cylinders. What are the neck threads size. 6" diameter is probably a bit much for triples.

They are standard 3/4" valves.