Get the part number for the nut then look it up
14-0303 aka 3/8"UNF aka 3/8"-24.
Fwiw, I fit 3/8"UNF stainless dome nuts to keep crap and water out of the grease nipple (and thus the brake cam). I replace the large o.d. washers with stainless so they don't go rusty where the nut scratches 'em.
Dome nut fitted loosely over the nipple can be tapped with a hammer to release the lever from the cam without damaging the nipple.