There are many different types of valve outlet connection.  The two basic types are 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.

Depending on where cylinders are sourced different standards apply, in the the UK it is BS 341 but German DIN 477 

and American CGA are also available.  The only way to be certain is to check the gas company's (Messer, Linde, 

Air Liquide, Airgas, Air Products, Calgaz etc) data sheet for the 

given cylinder size and compare that to the manufacturer's data on the gas control equipment used.