I got my 686+-5 straight from S&W to my front door. It cost me $300.00 new. I sent them my 586 with an AAAxxx serial # for a warranty repair and it could not be fixed. They offered the Mtn. gun as a replacement for an extra $300. They were new then and they asked me if I wanted 6 or 7 shot. Having been unaware of the 7 shot, I took it. So yes, at least a few 6 shot Mt. Guns were made. They only made like less than 2000 before starting to put the locks on them. I had the black front sight on mine, but it has a 24K gold bead now. My 686+-5 weight is 36 oz. with Jordan trooper grips, and my 686-1, 6 shot is 40 oz. with Coke bottle grips. The trooper grips are heavier so a Mt Gun is 4.5 oz. lighter. Bit the balance is much better than the full lug. And I get a stronger gun with lighter weight than a Model 66. This is a NEVER SELL GUN.
Here is a bit of trivia, when I got my 586 in 1981 they were new and there were no speedloaders for them. Had to have for duty use, and found speedloaders for Colt Python were perfect, and still in use today.