If I understand correctly, the rib is brass covered with a thin layer of ferrous metal. An aftermarket 'heavy' rib being the purpose of the brass I suppose. You're right I can't see it that well, but I'm perfectly happy to take your word for it.

The builder was careful (or lucky enough) not to sand thru the ferrous metal sheath (unless solid at that point as you suggested) when he ground the muzzle and front end of the rib/sight base. Which means crowning the muzzle according to my advice might be a big mistake if it exposes brass.

__________________
Jim
S&WCA #819