It is required in the U.S. by OSHA and the Compressed Gas Association (CGA) that cylinder valves be designed differently to prevent interchangeability with gas systems and equipment. This is one of the most important safety practices in the gas industry. For instance, if a used acetylene regulator were allowed to be assembled to a full oxygen cylinder, the result would be a catastrophic fire/explosion with the possibility of severe injuries and/or death to personnel in close proximity.
Every cylinder valve has a number stamped on its body to indicate the proper gas service. That number must correspond exactly to the CGA cylinder connection number on regulators and valves, or it will not assemble. It is extremely dangerous to attempt to connect gas regulators to cylinders for which they were not designed. All users of gas control equipment should make sure that all regulator, pigtails, manifolds, etc., have matching CGA numbers on the cylinder valve. If there are any questions at all, immediately contact the gas supplier or Harris Technical Service for instructions.