Windshield wiper knob
I have a couple spares, but both are missing the little clip that goes in the slot to hold it snug. It's just a thin piece of brass. Just about anything would work. Maybe even a piece of plastic from a milk jug.
One is pitted pretty badly. The other is slightly, but it wasn't too noticable in the car. The flash makes it look worse than it is.
The pitted one is $5 including USPS. The nicer one is $10 shipped. Let me know if you want one.
The part number is 2864411. That helps with Google and Ebay searches.
If you need a really nice one, Brad's is showing NOS for $39 plus tax and shipping.
2864411 - Brad's NOS Parts