On the Springfield Armory prototype Krag, the rear of the sight-base appears to clamp a flange on the fitting attached to the front of the hand-guard. That may explain the need for a more robust rear-screw.

That tall rear screw-head on the prototype does not interfere with the 'hump' on the sight-leaf. But, if the leaf is put 'up' in the vertical position, pinching of a thumb might occur.

A flush rear screw-head makes sense on the form finally adopted. It would eliminate a potential dangerous edge.