It used to be that the factory would not change the finish on a model 28. It appears that the hammer and trigger are also plated, if so, it is doubtful that the factory did it. It was their policy to leave them case colored. So, I would think it doubtful that the plating is factory done. They also left the extractor blue on nickel plated guns. Hope these clues help.

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Richard Gillespie
FBINA 102