Art wrote:
Every one I ever saw had natural metal wings. And I saw a hell of a lot of
them

Well Art, you didn't see them all.
The P-51D came from the factory with the first part of the wing, about 50% of
chord IIRC, painted. The panel lines and fasteners under the paint were puttied
and sanded. This was done to preserve the qualities of the laminar flow
airfoil.
The paint was aluminum colored which probably gave you the impression of
natural metal.
At a later point in the production runs the putty and paint were deleted.
As most of the aircraft went through depot maintenance the paint was removed.
By the time the Air National Guard got the P/F-51s (1946 or 47) there was no
wing paint required.

Rick Clark