39-2 and Nickle, Too
I've got a couple of "all-nickel" 39-2s as well as spot blues, too. The lower one goes in the serial numbers the more likely it'll have quite a few of the nickel parts in question, if not all nickel parts.
As is the case with far too many 39-2 "factoids" the certainty of "when" bleeds away in the face of time, but S&W did produce all-nickel 39s then at some point following the changes wrought with 39-2s the all-nickel gave way to the "spot blue" (for the lack . . .) nickel pistols.
Furthermore, as far as can be determined, the switch to blue was done piecemeal, e.g., first came the sights, then the trigger, and so on (this correspondent suggests the preceding "order of things" not be taken literally, because if one knew the order it all came about, then one would likely also know when such happened).
Longtime S&W Fourm member Cougar015 has nickel 39s on either side of the Great Divide and will likely have as good of a handle on this as would any. Hopefully, he'll chime in at some point.
Later.