I thought someone would have piped up an answer quickly, it had been so long I couldn't recall exactly, digging through some junk and few boxes to get access it looks to be a bolt head. I can't feel a nut under the fender, but it may go right into the upper fender boxed section and be a self tapper.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.