Here is the badge without its postwar coat of silver spray paint. Removal was relatively easy, if not particularly fast; the oxidation of the base metal of the badge and the poor paint preparation made it feasible.
If you look carefully at the badge, there are still a few traces of the original silver wash visible... mostly small "flecks" here and there, but a bit more in areas like inside the oakleaf below the handle of the grenade, the upper details of the right wing of the eagle, etc; this in addition to the wash that was still very strong on the areas of the reverse (where the hardware was soldered). Some of the remaining wash is still silver, but some has gotten a "bronze-ish" tone.
The obverse of the badge is as dark as the reverse, which makes me believe that Stewy is right on the money: the silver wash was thin to begin with and has long been absorbed by the zinc.
(A personal aside on this phenomenon: back in 1972, my school allowed a one-day display of all the WWII "bring-backs" that my classmates could gather; many of the zinc badges to be seen had already gone very dark, others still looked pretty good. Zinc pest was already in evidence on some.)
I wouldn't normally touch a badge, but in this case the circumstances were right. IMO, there was some nice detail lurking under that cheap makeup.
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