Reporting back on what I found in my records and pictures.
First needed to sort of ignore the VINs and focus on when the examples were finished/built at SA. With this it sort of tightened up the group
Of the examples I have with the bolts not all of the head markings were visible on all examples but in others all six were clear enough to make out the markings.
My earliest example had SEMS head markings and it had a SA "Build date" (according to the Registry) of 10/10 so a very very early example that may explain the different markings
Of the others I found with the bolts the following four examples were found with head marks of RBW with and without circle head designs. Without circles beign found on the last one I have right now.
Examples ended up being from each months covering four months. Nov - Feb
In order these cars were built on
11/6/66 - 12/14/66 - 1/4/67 - 2/16/67