Gearbox Top bolt Size

Hi there

Can anyone help me with the thread size on the gearbox top bolt

diameter is 9/16 but thread size Hmmm...

I believe there are two types with different threads.
One is the SAE 9/16 x 18 thread per inch; this is the hex head bolt and captive round nut on drive side
The other is 9/16 x 20 tpi used on early bikes; captive round head bolt inserted from drive side and nut on timing side