... a Cavalry troopers main 'mounted' armament was the Saber. The saber was worn on the same belt as the pistol and goes on the left side. reins were held in the left hand and the saber was cross drawn with the right hand. Having the pistol on the left side would make the pistol and saber bang around as you either rode or walked. Butt forward right side is also very good when mounted on horseback. It doesn't interfere with the reins, is very comfortable, and the jostling around of a fast canter tends to make the gun move 'into' the holster not out of it. Very important on a march if you forget to button down the flap.
The rearward angle insures the barrel end doesn't rub against the leg when fully seated in the saddle. When you look at old pictures or accurate illustrations you will see the holsters were made to sit with the barrel angled backwards at a sharp angle. If a trooper did need to pull the pistol when mounted the right hand can grab the butt and pull the gun out easily without having to struggle to get that long barrel out. More than just hold the gun in the flap keeps the gun dry and keeps dirt and mud off the gun. In the field skirmishing that flap keeps the rain off of the pistol.