They look like inflators for life vests or rafts. That's about the
only thing I can think of with that small a capacity. The thread is
probably British, but might be something proprietary if it's
ex-military.

You might want to rethink your power source. Figure out how many cubic
feet of gas you're going to get out of the volume of liquid CO2 that's
contained in those cylinders. It probably isn't as much as you'd like.

Stan
Stan