finding the right size thread/diameter of a bolt when you don't know
Plan B: Put an adjustable wrench on the bolt/stud threads to measure diameter, remember if the threads are course or fine, take the adjustable wrench into the store, find a bolt with threads that fit into the wrench, and pick out nuts with course/fine threads (or three of each, and return the rest later).
Whichever screws onto the bolts is the right choice.-Bob