I have a couple of theories about the butt forward carry. Just my opinion, neither may be correct.
For right-handed mounted cavalry troopers, butt forward left the right hand free to wield a saber. For right-handed cowboys, butt forward on the left side reduced the possibility of fouling the pistol with the lariat. When I was a kid on the ranch and carrying a Ruger Blackhawk strong side while working cattle, I looked down to see the revolver fully cocked. Probably from entangling my rope with the hammer. I went to a holster with a strap that held the hammer down.