There is supposedly "some" evidence (maybe") for a wedge inserted under the top edge of a grip wrap. However really old bows have an ultra low survival rate and cord or leather grip wraps are even less. So finding actual provable evidence is nil as far as I can ascertain. It certainly seems like it would be a logical step as would grip wraps, from a functional role rather than a decorative one, if only to help maintain a proper consistent grip location for the optimum dynamic balance between the limbs.
I suspect the shooting off the fist was common for the eons, with archers protective gloves and gauntlets developing by the early medieval period. My personal suspicion is that the inserted wedge shelf probably developed in the archery sport revival in Britain after the war bow and the career military archers with gnarly hands and monstrous shoulders gave way to a more effete archer who's hands required more protection from shafts and fletchings.