Google is your friend. Looks like it might be a cryogenic tank used to store Argon gas.
http://www.hse.gov.uk/cdg/pdf/standard/dot4l.pdf
Page 13 explains the markings on the tank.
My continued reading of the doc shows it could also be used to store liquid oxygen. Either way it looks like a really nice tank that could be used...after thorough cleaning.
I tried to call the guy but got no answer. I was going to ask him if he knew what it had been used for previously.