The J frame dots should work on a fixed sight K frame.
While the J frame's front sight has a step up right in front of the top of the serrations, XS ignores that step in their measurements. XS considers the top of a J frame's front sight to be the top serration (.155" above the base of the sight).
K frames don't have that step at the very front of the sight. The very top of the front sight is even with the top serration and is exactly .155" tall measured from the base of the front sight. Same as a J frame.
XS states in its installation paper that the center of the dot will be at the same height as the top of the J frame's front sight (.155") so it should be at the same vertical location in a K frame.