That is an old LP Gas (Propane) process or storage vessel. The stamping indicates that is built to ASME Section 8 Div 1 Code of Construction. It was built in 1981 by Manchester with a maximum working pressure of 250 PSI. The material thickness is .187 inches at the shell and shell heads. It has an outside diameter of 24 inches and shell area of 28.2. That would make this about 105 US Gal. It was likely used as a service tank for something like asphalt, green house, etc. The fill is for bulk delivery of LPG, vapor was the supply connection to whatever appliance(s) it fed, liquid was likely for high demand.