Update: The nut is 1/2 inch, BSF threaded. In the end, I found the bolt is actually a threaded rod with nuts on both sides; could have easily slid it out and placed with metric of UNF. Nxxt time... I found the hardware at a shop called Morries PLace, in Ringwood IL. Vintage European bike shop. Man, its a candy store and I was drooling all over the place. BSA, Duc's, Rickman, Truimph, Norton, BMW, Laveria, Panthers, Matchless, AJS... the list goes on. Found a wonderful old RE v-twin. Im thinking 1930s.
Here are a few shots from this place. If you're in Northern IL, check it out!