Optional since the early 1950s, and I would have guessed that it became standard in 1975 when they made the target hammer, trigger and stocks standard but based on my 1977 27-2 apparently that would be incorrect.
I don't recall seeing a RR/WO on any Mod. 27-2 that wasn't a special order. That goes for those shipped in the blue box or the presentation cases. They and the Mod. 25's were the only "premium N frames" not being shipped standard with those features, as were the Mods. 28 and 58. Standard sights for the 27's were Baughman or Patridge front sights. Patridge for the Mod. 25's...