There are horizontal propane cylinders as well so you need to know what you have. If the container has welded on brackets for permanent mounting it is a Tank, if it just sits in a holder and you can take it out to refill, it's a cylinder.
As beaslwd said tanks do not need OPD valves, cylinders do.
In case you want to buy a horizontal cylinder they retail for $255 and are made by Manchester Tank. The product number for the 20# one is 1175TC. Mfg in accordance with DOT 4B, 4BW, and 4E specs. Fill valve and pressure relief are on the end but on top of the end.
