Jay Rolfe

I have an old U-built trailer that probably has chrysler brake drums on it as the right side has left hand thread studs. I have lost two nuts that are 13/16 size but I am not sure what the thread size is. Would you have a thought? Purchased some 1/2"-20L and they were too big. thinking that the standard size was 7/16" studs.
Any help would be appreciated.
Jay