Move this to 68 section since it is focused on one year only and also a single plant. As mentioned in the past I would not suggest using AMK as a guide to correct head markings for specific cars.
From my notes, for that type of "sems" screw (AMK Guide to Ford Fasteners - pg 32), the washer does not have teeth or serrations but it is slightly dished. It also has a sharp edge on the edge of the dish that goes against the item to be attached.
Looking at what pictures I have San Jose started out using the double hash "" head markings on these bolts. Moved to the SEMS for a short period with the large "T" for lack of a better description in words then moved on to the "23" head markings that you show in your post.
The "T" is part of the grade 5.1 marking, which was implemented in 1968 Ford products.
What does the washer and the lead point look like on the screws pictured in the first post of this thread?
Jim