I have my doubts about it being a bell holder, as the ends would be more solid for a swivel joint. I am asking my experienced wreck diving colleaugue who will probably identify it (boiler relief valve ?). I have just shown him the photos and he agrees with my assessment and says "Tend to agree with you, the ends would be circular to swivel, and the bracket would be deeper, also not sure they would use steel. It could be a pressure cover holder, that could be removed along with a cover to see inside a vessel." So there is our answer . This is not a bell mount but something else.