Spares Manuals, for A65/A50's anyway, indicate a standard thickness (1/16" or .0625" or .064") for the studs and bolts, with no washer indicated for the 3/8" short center bolt.
All of the head bolts for both of my A65s have always had what look like original washers, so I had to look at the '67 factory Spares manual, which I've owned for fifty years, and what I found was strange indeed:
One of the two rear head bolts is shown with a washer, page reference 13, P/N 2-923, and the book indicates that there are two of those.
One of the external studs is shown with a washer, page reference 15, P/N 2-2138, and the book indicates that there are SEVEN of those. That would account for the four studs, the two FRONT bolts, AND the center bolt!
This begs the question, why are the two rear bolt washers different from the rest? On further examination, I see that the two rear bolts are part number 68-338, and the two front bolts are part number 68-337. The four head bolts are the same length, so this difference in part number must be a hangover from 1965 when the two rear head bolts were 5/16" diameter! Note that that would make those two flat washers 5/16" diameter as well.
In any case, a washer on the center bolt IS indicated. Gary, where do you see washer thicknesses? I don't see that in my Spares book.