Just an F.Y.I. I restored a 66 Shelby carryover car a few years back, was built Aug. 10, '65 at SJ. It had pink paint on the top (as installed in the car) of all 4 "U" bolts. I know the build date was accurate, because I found a quality control line card under the original carpet, passenger front floor.
Hope this helps to add evidence to a possible pattern.
Ken Mann