I have been looking around for a pressure reservoir to build my Poland Spud compressor.
I called the local fire safety company and enquired about co2 fire extinguishers.
They apparently throw them away when they are out of date. I could have them for hauling them away.

When I went to pick them up this morning, I was asked if I wanted empty ones, or full ones.

I removed the valve from the empty 5lb and verified that it does have 1/2" NPT threads.
These bottles have an 1800psi rating, and are steel. (heavy 13+lbs empty for the 5lb)
Then I noticed the swivel on the nozzle. Once you remove the nut, you have a ready made probe style fill, with 1/4" NPT threads.

The 10lb looks to have 1" NPT threads and even comes with a pressure hose.
The main valves could be used to replace a blow gun maybe?
Other details:
These are old General brand extinguishers.
The 10lb weighs 20 1/2 lbs empty according to the stamp on the valve. +10lbs when full of course.

The pressure hose has a rating of 2500psi on it.