HI Luis i think if they look like a fine thread it is 26 TPI CEI ( CYCLE THREAD ) or if it looks like a course thread it is BSW ( WHITWORTH ) that is the common threads that i use on most of my Ariel's some times i find someone has used different type of bolts and screws if they did not have the right ones like the ARIEL i am doing now it got lots of all different types of nuts and bolts with the wrong threads and nut sizes
Here are the two nuts from my WNG, as far as I know they are original. I had a problem in that with the pannier mounting brackets fitted I could not get a spanner on these nuts as they were too big. I then used two half width normal size 5/16 nuts with washers for the raised part to solve the problem.