BTW- About 99.9% of the 4" 27-2s were made 1979-1981. Prior to 1979 the 4" length was a custom order and is very seldom seen. I have seen exactly one (made in 1963 on special order and inscribed for presentation) in my life. In 1979 the 4" replaced the 3.5" as a standard length.
Also, in 1981 S&W must have been running out of the pinned barrels for the 27-2s (and some other models, too) and a large number, most of the ones I have seen, in fact, have the unpinned -3 barrels. I call these the 27-2.5 models.