Most likely Jim was thinking of the 67-70 perchs which had zinc phosphate bolts.Been there done that.
I recently removed two sets of the spring perch bolts, one set from my 66 "K" car and the other from 6S19xx. Both cars are San Jose built and the perches were factory. Both cars have been garaged almost from day one, eg minimum road damage. When I dissassemble anything, I look at the condition including finish (credit Jeff). Neither of the two sets' bolts had a trace of cad or zinc, especially the underside of the bolts heads' and the serrations. The only part number I could find for 1964-66 perches was on Ford Car Parts ILL 30 page 04 identifing them as 373134-S, discontinued (aka, not serviced). I measured mine and they are 3/8-24X7/8 hex head grade 8 marked "Rockford". I have no other documentation on this bolt. For 1967, and on including service, Ford used a 3/8-16X1.0 hex head grade 8 marked "C" and "M" (I have two sets on the shelf) identified as 381792-S2 (see AMK Ford Fasteners page 121 and Ford Car Parts Sec 30 pg 24). When I replace the spring perchs on 64-66 Mustangs, I replace the 3/8-16 bolts with 3/8-24 bolts.
Those are my observations.