I'll bet you 10 donuts to a Quater, those are just regular nuts, commonly used on "split ear connectors", electricians use. Take the dimensions you get with a tape, and go to the commercial electric supply house in your area and buy the same size connector, and use the nut you get there. You might check at HD or the other place, but they might not have them that big. If they do, they will be cheaper there for you.
Use Noalox on the threads, when you assemble that stuff!