I don't know that in 1740 there were all that many Indians firing from horseback...
The two finger preference may have been just that - a preference. Even then, how widespread was this preference? Trade guns were distributed over a wide swath of North America.
I have seen many Inuit in the Eastern Arctic using the middle finger, rather than the index finger, to fire their rifles.
Whatever the reason, the fact that the large trigger guard was a standard for over a century would indicate it was preferred.