Various production changes and serial ranges or prefixes can help put it within a DOM range.
One thing to be aware of for complete disassembly of cylinder is that there can be either right hand or left hand thread on extractor rod.... the knurled rod end on more than one 27 has been buggered with attempts at unscrewing the wrong direction!

1960 (27-1) change from right hand thread to left hand thread.
1957 model changed from "357 Magnum" to"27"... et al. 1982 cylinder length shortened and counterbore deleted. 1948 to 1975 2.65" trigger. 1975-1980 .50" trigger. 1961 eliminate trigger guard screw. 1968 eliminate diamond grip. 1969-70 change from S to N serial prefix. 1979 eliminate 3 1/2" bbl. ... and so on for more various changes until model discontinued. But it appears this one is one of the earlier 27s. Also, 1991 re-intro of 3 1/2" bbl. 1994 Model 27 discontinued.
I would look closely at reblue/refinish- kind of looks like the grips show a wear while the metal doesn't. Grips could have been changed to earlier diamond grips or the metal has been refinished??
Standard Catalog of S&W by Supica is probably best reference resource.
No matter... IMO well worth the $300 asking!