Well, it's certainly a puzzle to me....
The old gentleman from whom I bought the William English canoe insisted that it came along with the canoe, which he purchased in about 1948. I can see how an early, small outboard might attach to the mount with the lower cradle meant to counter the thrust of the propeller. This would be necessary since the green clamp area is not fixed in position- it rotates around it's attachment bolt (where you see the large hex nut). Perhaps it was meant for another type of craft. I do agree with your comment about the contact with a thwart, but I'm assuming that a user of this mount would likely have altered a standard thwart or made a dedicated motor mount platform to use this mount. Maybe it was made to mount on a canoe (or other small craft) with a long, flush stern deck.