There would be a market for the magneto which would fit most Enfields and the specification plate would certainly be desirable to someone building a late 1940 WD/C.
Yes, brass plate.
It is just the mag, no dynamo.
Not sure where I'd track down someone rebuilding a WD/C, it would be better on the motorcycle than on the Lister engine it ended up on.
I'd suggest a post in the "Sales" section here, preferably with a photo or two (it requires a separate photo host though but we can talk you through that or post a photo for you).
With a bit of luck, someone with a contract C6215 WD/C will be looking for this very magneto.