There have been a few posts lately regarding the finish on the slide stop. It seems the black component of the HE finish wears pretty easily on the slide stop. The actual rust inhibiting properties aren't affected however as the coperish color under the black top coat is what prevents corrosion.
On my HK45 it seems the hammer has a polymer coating over the rear cocking edge to aide in getting a better purchase when cocking the pistol manually.
To answer your last question, yes... HK does use MIM components extensively. While I always prefer tool steel components, I don't recall any problems with HKs MIM parts. I do recall Bruce Gray mentioning that the extractor would be better if it were tool steel.
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