just doing some google fu but cant find anything about markings, its been a while since i remember looking into it but i vaguely remember that the markings are totally different to bottle rated for air.....i defiantly do remember that some of my 3.5s did have DOT markings but that it also had CO2 stamped into them, possible that the dot rating for co2 is acceptable where are the dot rating for an air bottle isnt.....will try and root up some solid info, if i cant find anything will email Bully he will know for sure.