In the manufacturing process, roll dies became dull, wore out, and could have been applied by a different hand at the machine resulting some heavier and some lighter. I too have noticed differences in many of my pre-27s. Not a sure sign of a refinish. Also, remember in the 1950's the guns were still being hand fitted prior to bluing, finish polishing and final assembly. The polishing could also have impacted the roll mark depth and amount of metal displaced by the roll.

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Richard
Engraved S&W fan