I recently bought a Matheson regulator that came with a CGA stem but it doesn't seem to connect to the valve that is on the tanks. Below is a pic of the valve connection and my CGA stem. Any help you guys can provide is greatly appreciated.
You appear to have a CGA346 stem and nut (air), a CGA350 stem and nut (hydrogen) or perhaps a CGA540 stem and nut (oxygen). They are all very similar looking. You will need to replace it with a CGA320 stem and nut.