Leon,
The diamond section copper ring was introduced (I think) part way through OB production. Early OBs are illustrated with flat copper wrapped asbestos ring. But I have seen plenty of OB heads with the grooves for the diamond section washer. I have seen those in both copper-asbestos and solid copper. The Works bikes in the thirties reverted to ta spigot head joint.
I make my rings from square electric motor armature wire. Welded slightly small and stretched over a mandrel to make it truly circular. I don't bother to roll the square on edge to make it a diamond section, they seem to seal fine in the flat, with the diamond grove crushing the edges of the copper square wire slightly. The square section does need to be a little smaller than that of the diamond joint rings. I found 0.127" (3.22mm) square worked well, about 0.131 (3.35mm) before the insulating varnish is removed. You want a slight gap between the head and the cylinder, to ensure the joint is sitting on the copper ring. I have heard of folk also using round copper wire for joint rings, but to my mind they would be harder to file the weld joint accurately. Though once you anneal and put the crush on it, it all evens out!
Rings after welding.
Welded, joint filed, annealed, and stretched on the left. Cut and rolled rings on the right, still with insulating varnish.
-Doug