Hi Mark
The little sketch doesn't specify a hex head and as someone had mentioned just slotting it i assumed 20.5 Dia not A/F(my bad).
1.75mm & 1.25mm refer to the pitch, the effective diameter has to be within a given tolerance for the type of fit required or the thread with either be too slack a fit or will not fit its matting component.
There's a little sketch in the ZEUS handbook that will explain.
I wasn't trying to be clever, i was just interested how an accurate thread could be produced with limited equipment, with a CNC lathe it shouldn't be a problem.
I would consider a ring gauge to be a proper gauge also the thread parallels, these are required to find the effective dia, in the case of the ring gauge you have a go-no go situation, if it fits and isn't a loose fit it's acceptable,with the parallels you do get a definitive size. just want to be clear that i have zero knowledge of CNC turning/milling ect
Regards