The round butt N frame was 'sort of' sized to be much like the round butt K frame, because that grip size has proven over time to be "so right" with shooters, but they aren't exactly the same. Notice how much of the frame near the top of your gun's grips are exposed.

You can put a set of standard K frame square butt target stocks on a square butt N frame and they will 'sort of' fit the same way, which is to say, not quite right.