Frank,
The bolts used on 6S477 ( one of the earliest to have a Mico) were , two non plated, 9/16ths wrench size . 3/8ths coarse by 1" long on the upper two holes and a ''header bolt" 7/8ths thread length , on the bottom hole. It is important to note my car along with MANY others had the mounting bracket changed "under warranty" to an "improved design" @ High Performance Motors. Sooooo, the question "could be" did a mechanic @HP use a header bolt on purpose or was it done at SAI? I have seen it on several other cars so I know it wasn't a fluke. The header bolt is the same style as the header bolts supplied by Cyclone ( not Rockwell) at the time. Those masters were installed at SAI so some bolts had to be added by SAI over those assembly line furnished bolts. I would like to hear what other "un molested" cars have. I say unmolested as the header bolt could easily be mistaken as "previous owner/mechanic added".
Randy