Do you mean one of the little "ears" or bumps that is cast into that fitting that the pin slides through on one side of the boom, and locks into with the pin on the other side of the boom?
I would suggest disassembling it from the boom and taking it to a welder. (I believe mine is held in place with 3 stainless screws...another project if they've corroded to the aluminum).
If you have the piece that broke off, a welder can attach it, and if not he can (with some difficulty) build up replacement metal.