Forgive me for sounding like a spoilsport, DOC, but high pressure gasses can become serious dangers.
I've probably told this tale here before, but it seems appropriate to tell it to you. I lost a SCUBA diving buddy back around 1960 when he used a cast iron 3/4" NPT to 1/2" NPT reducing bushing to adapt a SCUBA valve onto a CO2 fire extinguisher bottle. He likely would have been OK with a brass or steel bushing, but cast iron wasn't up to the task and the threads sheared off while he was filling the tank. The 1800 psi air blew the valve up through his chin and into his brain.
Whatever you do, If those 47 year old tanks haven't been recently tested, they, and whatever adaptors you end up with to mate with those threads, should damn well be recertified by a qualified place before you fill them with CO2. That should include hydrostatic testing in a vessel which will measure the "expansion and contraction" of the tank. Fire extinguisher shops have that kind of equipment, that's where we used to take out SCUBA tanks.
Just my .02, but if it was me I'd consider retiring those antique tanks and buying some new paintball gun cylinders for your combat robot, the fittings for those are readily available, and they're made to stand up to the rigors of being bumped and dropped by the paintball players.
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.