I suspect that moses is correct about the intended purpose being to deflect upward [or forward w/the hammer down] any escaping gas. For perhaps a definitive answer, an inquiry to an arms museum's curator w/picture and added arrow [since I know of no official term for that feature] may produce the best answer. There may be terminology for that feature as well, I've just never heard it.
Joe, I've always heard those highly carved snails referred to as fences, but I may be misreading what is being spoken of.
