Previous poster was correct in saying that your no-dash gun would have the square front rear sight. These are no longer available from S&W. You may find one at Numrich, but eBay is probably the more likely source.
As was depicted in racer19j's pictures, the critical dimension is the distance between the elevation screw and the front mount screw. It is not the same on all S&W sights so you need to measure yours and then make sure the one you purchase matches that dimension.
As an alternative, if you are not a purist, Bowen makes their Rough Country rear S&W sight in a square front blank tang. Best installed by a gunsmith - you drill the tang to the correct dimension. The Bowen sight, in my estimation, is far superior to the OE S&W sight and worth the extra money.
Good luck with whatever you decide.
Adios,
Pizza Bob