Believe it or not, some of the very old engravings of soldiers with wheellocks and rapiers show them holding their pistols in just such a manner. At the time, however, the optimum distance for discharging your pistol was just about contact distance, so proper technique, at least as it has come down to us, was more or less reserved for your sword and dagger. Although the last such illustration I saw, which happened to be of Guy Faulkes, showed the individual on foot, it is conceivable that when mounted and shooting to the left, there may have been a natural tendency to tilt the pistol to the left but there may be some other reason more related to fencing. In fencing, as you probably know, you twist the sword arm to the left in a lunge and do the same when you assume the charging position when on horseback, provided you are armed with a thrusting sword. I'm sorry but my riding instructor when I was in college wouldn't let me try that out, not with one of her horses.
I had pretty much switched over to handguns by then anyway.
I had pretty much switched over to handguns by then anyway.