My experience of gun shops, shows, and other venues is that the 6" 586 in nickel is not an uncommon gun. Not rare at all. I'd say around 1/3 of 6" 586s I see around here are finished in nickel. I was just looking another one over a couple of days ago.
About a year or so ago I saw a 4" 581 finished in nickel, I went back for it the next day and it was gone. I would guess that would be a fairly uncommon L frame/nickel gun to find, as it was the only one I've ever seen.

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Geoff. Since 1960.