Charles Kennett of Newmarket England sought a way to concentrate more weight behind the ball. So he devise and patented an iron that had a thicker than normal blade but included holes in the hosel, heel, and toe. According to his 1928 British patent, the holes in the heel and toe could be left completely open or they could sometimes be filled with lightweight rivets of aluminum or other material to keep debris from collecting in the holes. 

Complete with its original shaft and grip, this example was made for Kennett by J.P. Cochrane & Co.  Seven of the holes in the heel and toe were made without anything filing them, two holes in the toe were filled with aluminum, and one hole in the toe was filled with some other material.  The back of the head is marked "C.H. Kennett, Links GC, Newmarket." along with 'R2J" and more.