A lot of these are variations of BSP. Bust out the calipers and a thread pitch gauge then look up the thread charts to figure out what you have and what you need. If the adapter is not common, you might need to make your own, either cut it on a lathe, or braze two fittings together. Two good sources are Mc Master Carr, and Maryland Metrics. They both have a ton of specialty fittings. Another option is to change the hose. You might get a match at the torch end instead of the machine end. 3/8"-24 is often used there, even if the machine end is M16*1.5
Long arc, short arc, heliarc and in-the-dark!