Interesting- - -
I had never known that S&W made a six-inch M15. Always understood that the M15 was just the K38 (M14) made with 4", and later 2", bbl, for ease in carry. It was cataloged as the Combat Masterpiece, and, of course, was only in .38 Special. When they shortened the barrel, they also made the rib atop the barrel narrower, to reduce weight for everyday holster use.
The saga of the Combat Magnum is well known--Just a M15 with longer cylinder and under-lugged barrel. They widened the top rib, too, presumably to reduce muzzle flip with the magnum loads.
Some years ago, a friend who ran a security guard company in Dallas area bought several K-38 4" guns. I at first thought they were cut back target guns, but, no, on close examination, these were factory barrels, wide rib and all. About the only thing we could figure, was that this was part of a run for competetion in "Police Service Revolver" matches, where standard-length guns had to be used. Extra barrel weight would help . . . .
I didn't know this "cross dressing" also went the other direction.
Learn something new every day.
Best,
Johnny