I would like to chime in here, I specialize in the Plains bow but am very familiar with other regions. Here is my thoughts........ The average bow did not have a handle, shelf or mark indicating where it was held. The holding of the bow was a natural occurrence and each man knew exactly where his hand needed to be when he was ready to shoot just by instinct. Having made and shot short bows for decades I know in an instant when I pick up a short bow where my hand needs to be placed, if it's not right you can feel it right away!
It is noted in many writings that Indians didn't fight with bows in the rain, The simple reason for this is that the bow strings so often made from sinew or rawhide would stretch. Handles were seen occasionally but it was not the norm for the Plains people. The highly decorated bows and ceremonial bows are seen portrayed with handles, Most horn bows had some kind of handle even if it was a full sinew wrap, this was because alot of these bows were spliced at the center.
I have never seen a southern Plains bow with a handle of any kind other than a wrap of sinew, and that was not used as a handle per-say but rather to hold sinew backing in place.
E