Metal Hardness and Finish
The topic of varying colors on example Lugers is often brought up, with the side-plate and toggle usually being the area of interest. Pistols often exhibit varying colors in these two areas. .
I came across a couple of photos of a "dug" Luger, which apparently rested in a largely anaerobic atmosphere. Observe the difference in finish/surface deterioration of the various sections of the pistol, with the oft mentioned toggle and side plate being prominent.
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