Hi Roy,
I wasn't aware that Douglas used copper/asbestos washers under the exhaust valve springs. It seems a little weird as washers of this type - because of their 'crushabilty' are usually used where a seal is required. The common application is under the valve cover bungs.
That said, many early engines used a spacer of some kind between the very hot part of the cylinder adjacent to the exhaust valve and the spring itself. This was often made from very thin pressed steel, shaped to keep the spring maybe 3-6mm away from the cylinder, to try to minimise heat transfer to the spring. The problem with most copper/asbestos washers is that the copper usually surrounds the asbestos (at least on the inside), and because copper is an excellent conductor of heat I'm not sure whether it would do a good job of isolating the spring from the cylinder heat.
I wonder if part 482D was a "sandwich" of copper/asbestos/copper, with the two copper sides not joined. At least something like this might help to keep the the spring a little cooler. It would be fun to see an original!
All my waffle can possibly be ignored - modern valve springs can probably take the heat without sagging, even without a spacer.
Leon