Does your keg look like this?
Those are McMenamin's kegs, and they use Golden Gate. As far as I know, those rings don't come off, and all cleaning and such is done through the bunghole. (lol) That wooden bung is usually broken off and another one is installed after beer has been put back in. A rubber bung may be OK, but I'm not 100% sure on that. Also look up Hoff-Stevens kegs, they are similarly made.