The main thing is that it has a good data plate. I believe anything after 1956 has a liquid withdrawal valve. Some of the older tanks the liquid comes out of the fill valve. If you buy a used tank if may be a good idea to make sure the (check lock valve) or bung as you call it is new. I have seen alot of tanks that the excess flow valve wouldn't open. I think the newer tanks with a screw in percent guage is less likely to leak than the kind with the four bolts. There are some 200 psi rated tanks around they are still ok to use around here but wouldn't be my first choice.