Issues would be width, frame height and frame length. How close the dimensions are would determine holster interchangability. This is, of course, assuming equal barrel lengths.
I am also curious how close they would be to my S&W M15. The 686 is an Lframe which I believe was developed from the K to handle .357mag. What I don't know is if they simply used the same dimensions as the Kframe and simply improved the metal or if S&W actually beefed up the gun physically to handle the magnum cartridge.
And if they did add metal to the frame - how much? If only a little then K and L frames could use the same holsters: if a lot then that option is out.
John