At one time this was a 500 gallon propane tank.  But has been converted to an oxygen reservoir to contain 'bleed-off' oxygen from an insulated liquid oxygen tank which is commonly used by people who do a lot of cutting torch work.  An insulated tank of liquid oxygen has a pressure relief which keeps the pressure at not above 300 psi.  This relief tank is equipped with an oxygen style valve which means a torch's oxygen regulator can be threaded to the valve fitting which means a conventional regulator can be attached for supplying a torch.  It can mean that a torch user can save and use all of the oxygen supplied in a liquid tank that comes from a welding supply vendor.  I rigged this tank up several years ago, and saved a bunch of money by capturing oxygen gas that would have otherwise been bled off and lost into the air.  This unit can be a money saving gadget for anybody who wishes to not lose some part of their oxygen supply.  You can call or text me at 785 657-0007.  I can load it for a buyer.  Call if you have more questions.  Thanks for looking.