Length, diameter and all of the markings on the shoulder will help....
1. It should have a marking such as DOT, TC, ICC
2. It should also have a 3AL or 3AA or alternatively, an "E" or "SP followed by 4 numbers.
3. the 3AL, 3AA or exemption/special permit number will usually be followed by the service pressure. For example "3AL3000"
4. Luxfer and Catalinas both use letter prefixes to indicate capacity so any singel or two letter codes are very useful. the cubic capacity is also often listed such as "S80" for a luxfer 80 cu ft tank.
5.The date of the initial hydro test will be stamped on the tank in a month and year format (MM@YY) with the "@" being a stamp indicating the third party company that did the testing. Stylized A's C's and arrows are common as are N's and C's inside a diamond.
6. There will also often be a manufacturer code or name such a PST, M303, Faber, Catalina, Luxfer, etc
7. Finally, the company that marketed the tank may also be there such as "US Divers", "Scubapro", "XS Scuba" etc.
