I believe this is the information you requested. The rivets used on the four posts are M3 X 8, aluminum, round head, I do not have the DIN. The rivets used on the leather strap are M3 X 10 because of the extra thickness of the strap.
The outer diameter of the M3 flat washers are slightly larger than standard washers and measure 7 m, dimension B in your drawing, and the thickness is approximately
1 m. The washers are only used on the outside on the side posts and are clear, or silver, zinc plated. They are used on both sides of the strap.
When installing the rivets I use two punches which have had the end hollowed, or slightly dished, to preserve the round head on one side and to also give the other side a rounded head.
Here is a link to a photo album of three side curtains, one restored and two original.
There are also photos of the side curtain bag which I had made. I have never found an original Speedster with more than one bag but I use two bags so that the plastic is not marred.http://imageevent.com/356consulting/spe ... urtainsbag
I hope this helps, let me know if you need additional data points.