Old as Dirt!
Absent a photo, it is difficult to comment very intelligently, but you may be looking at a former mounting for a lead shoe that has since been removed. Lead shoes or a "sabot" were in vogue in the late '70's early '80's as a means of increasing ballast, lowering a boats CG and adding "stiffness". In any case, Beneteau cast iron keels are quite robust and easily repaired/reconditioned.My 1984 Beneteau First has a cast iron fin keel. The bottom of the keel has a void in it. The bottom of the keel currently has a flexible piece of metal (consistency of sheet metal) on it, and there is a split from front to back (again, on the keel bottom). Sorry, I don't have a picture to show as the keel is sitting on the rack and is difficult to get visual on it.
I am guessing that this void is due to galvanic corrosion. Here are a few questions:
1. Do you agree with my assessment?
2. If not, do you have any ideas what might have caused this?
3. Is this issue of a void common or very unusual?
4. For repair, should the material be ground back to the solid iron, smoothed, and then coated?
Thanks in advance for the help.