Ok Mr Frost Camaro, I tried to take some pix of the captive nuts on the fenders, but I have both an OEM and Repro fender extension which was easier, and the type nuts used are the same as for the fenders.
OEM nuts, from outside and inside (square holes in fender):
Repro parts nuts, from outside and inside (round hole in fender):
Gary / 69Z28-RS