Maybe you know what it is, but have you used one?
I set out to figure out why the “inside” part of the blade is the shape it is.
It is interesting to note that the original patent (US patent number 775134 issued November 15, 1904) doesn’t show anything other than the hole in the middle of the blade. My best guess is that cutting out the rest of the parts of the middle that are cut out 1) save on steel, 2) keep it flexible, and 3) make it fit into the holder better.
If you have a better or another idea, please put it in the comments.