"Rubber ball on chisel Makes it easier to grasp." Is the title of an article in Popular Mechanics, issued in 1941.
"Slip over the shank of a turning chisel, a sponge rubber ball will provide a good hand grip, especially when the chisel is used in long length's of time. The ball fits into the palm of the hand as the tool is grasped in the regular manner, and tends to relieve muscle strain."