What is this? I got it in box from a storage unit from ‘93. It was in a box of mostly antique stuff, yet it was one of the few things actually wrapped up.
Tooth off a sickle mower. Hay cutter in other words.
edit: From the string, looks like someone was using it as a plumb bob.
Googling sickle mower images, that doesn’t really look like a tooth from one, it looks too big.
Possibly a tip from some kind of fishing harpoon, or a tent peg.
Barry Weiss, is that you?
The string leads me to believe it is a bricklayer’s line. Here are some modern ones
I thought it might be a plumb bob, but looking at the 3rd pic in the OP it does look like a mason’s line.
I’ve put on plenty of them on 80 year old farm equipment, I guess I could have included a reference. If I wasn’t on the wrong side of the world, I could go grab one off the junk pile, heh. Here is a more modern version:
http://qualityfarmsupply.com/products/single-prong-malleable-rock-guard
Not the exact brand, but you get the idea.
Damn, I thought you were just throwing out a guess. You nailed it in 4 minutes. Obscure specialized knowledge of antique farm equipment, who knew?
That picture is very convincing. I guess it could have other uses, but it looks very much like your picture.
What exactly does this do?
That look EXACTLY like it. Thank you so much, and thank you ALL of you for your help!
Hahahaha I wish! That dude is BOSS. I spent the whole time at the auction making Storage Wars jokes though. 
A lawnmower normally has a rotating blade, it cuts grass and chops it up, which is kind of what you want. For hay, you want to keep the whole blade and stem in one piece, so a sickle mower just cuts it and it falls over.
For how it works, you have a rigid frame, either on an outrigger type arm or built into the front of a cutter. A bar with flat, triangular cutting teeth on it reciprocates along the frame, and those teeth above serve to both protect the sharp moving edges from rocks and keep out large items. The big teeth are bolted to the frame by the hole in the back, the reciprocating teeth move back and forth in that slot you can see along the bottom, which is what the sharp teeth cut against.
If you’ve read any stories about dogs/children, etc getting legs cut off by running in front of a hay cutter…
The teeth on the actual blade move back and through the slot in it and thereby shear off the stalks of grass/grain etc. It’s generally called a knife guard, not a blade tooth, which is the serrated triangular piece riveted to the blade.
I’ll have Victorian sex toys for $200, Bob.
Made me snort my coffee…