These wingnuts were often displayed on a card near the cash register. When a customer buys a new bike they were asked would you like wingnuts on that? They were a low cost, add-on. They do have a vintage look to them. I had wingnuts on my first 10 speed and it allowed me to remove the wheels without tools and without fancy quick release skewers.

There are a bunch on eBay. Just find a pair that you like with the thread that juvela suggested and you should be good. Since these are not Schwinn, it doesn't matter which ones you choose. At least that is my opinion.