This protective cap has been designed to make it obvious when a propane cylinder has been used.  The protective cap must be destroyed during removal.  It cannot be taken off and put back on.  This prevents tank mix-ups in cages where someone removes the cap from a full cylinder, puts the cap on the empty cylinder, and returns it for exchange.  The next person sees a cap on the cylinder and assumes it is full, and carries home an empty cylinder.  A broken strap means the tank has been used.  This cap is Yellow in color.  Blue caps are also available as part number ACM-528.