I have Dufour 29 of 1977. Based on my surveyor, it has to do with the cable that is linked to the mast, so when lightening strike the mast it goes through that and then end in the keel.
Which Dufour do you have? I mean which year and what size of keel? Any idea if the keel is encapsulated or bolted?
The membership association for owners of Dufour Yachts