Cylinder valve outlet connections tend to be of two main types: internal – accepting a bull nose or external – using flat-seated with a washer. Within these types there are variations of thread type, size and direction as well as outside diameter.
Regulator inlet connections are normally available in brass or stainless steel. Some fittings may not be available in brass depending on the properties of the gas they are intended to be used with. A good guideline is to follow the cylinder valve material, if the cylinder valve is brass, brass or stainless regulators and fittings should be acceptable, if the cylinder valve is stainless steel, brass regulators and fittings should not be used.