As far as I know the very old wood planes used a wooden or steel wedge, no hole in the blade. This thick piece would make a very good wedge and people used what they had. Its been a few years since I sold them and I havent seen everything ever made. I dont understand the hole. I dont know what reason for the cut out on top. Only the more modern blades utilized a slotted hole as far as I know. The small hole wouldnt do much good in the long run because as the blade wore down and was sharpened, it needed to be adjusted. It does look a little oval in shape but more like a homeade hole, not factory made.