I know the hubs had different threads in different years on the other side for the bearing retaining nut, and they may also for the speedo drive nut.
it is likely 20 or 26 TPI ,
can't you just measure the threads on the nut?
It's not really a nut, it's the speedo drive ring, with the square notches. I don't have many thread gauges, just wondering if anyone knew what it was. The threads inside the hub are a little boogered.