I think there was a .22 Combat Masterpiece, presumably some M-18 variant, with a four-inch bbl. and a full lug.
I have a Cobra Gunskin holster that seems to have been molded for such a gun. Could theoretically be for M-586/686, but it fits a M-66 just right and doesn't seem loose.
Of course, my M-66 doesn't have a full lug.
I don't at present have a M-686, but think I tried it in the holster when I did have one, and it stretched the leather. So it's prob. for a K-frame, which fits right.
The holster is a very nice one, in great shape. I think I found it at a gun show. Only one that I've seen molded for a full-lugged K-frame. I doubt they'd mold a holster for a gun that didn't exist.