I have also seen the portion of the bracket that unbolts and holds the rubber as natural or zinc phosphate on some 67--70 examples . Those were all on Shelby examples which used the bigblock type base bracket and not the one pictured so I don't know if it would apply. I thought it may be a possible variation because the portion that holds the rubber is common to both. On the ones I observed it was more the exception then the rule. I don't mean to confuse the issue but just wanted to relate some other possible variations to the finish. Bob