Keg with weird cap on it

[snwtricky]

Hey guys. so my roommate came home today with a older up beat keg that he "found" lol. anyways we were taking a look at it and comparing it to our other one and on the top where you would tap it there is a strange metal cap. i tried searching google for something like this but found nothing. So if anyone has encountered this before and knows how to remove it etc. that would be great help.

thanks

 

[Retarded_Penguin_Brewing]

pictures might be extremely helpful in helping to identify what ya got buddy

 

[boo boo]

Yeah. it might not have been used for beer.
Pictures would be nice.

 

[KYB]

A big keg? If so, does it look like this on top?