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While not really rare to find this early variation with a different milling patterns, these are still around but not common. The early bolts were never withdrawn from service (Billy Pyle) and still can be found today.
The milling patterns are different from the later early bolts and of interest to the collectorAttachment 124977Attachment 124978Attachment 124979
Interesting that they did not Rev the bolt drawing number for the different milling patterns. Perhaps they had traceability through the heat lot numbers. Is there any significance to the number stamped above the drawing number by the hardness test dot?
C1 is the first heatlot used in production then J1. Both of these bolts have the early milling. J2 bolts changed to the wider milling. D1 bolts came after J2 which shows that heatlot codes were not always in alphabetical order.
The small number stamped on the end of the bolt (mostly seen on some J1, J2,and D1 bolts) is thought to be the Rockwell number (hardness strength).
Last of the revision 1 bolts had both the drawing number and revision number on the same line. Last ones also had SA.
Center bolt in photo has the drawing number and revision number in a singAttachment 124986le line