I seriously doubt the 1858 grip was designed to be held pinky under. For the most part I suspect it wasn’t designed with any aim except to imitate the Colt grip which came before it. They just got it slightly wrong. Notice Remington ran far and fast from that design to the much improved versions of the 1875 and 1890 which allow plenty of room for the middle finger and don’t require pinky under grips. If anything they overcompensated with the 1890 grip which has an awful lot of real estate twixt the grip and the trigger guard… it works though.