Good afternoon all.
I would say that it could be a 2mm or 2.5mm pitch thread. 2mm is neither ISO metric fine or coarse for a 20mm diameter. Those being 1.5mm & 2.5mm repectively. 2mm would be typically Japanese! (They have a habit of doing such things).
It's almost certainly metric, due to the flat crests.
It would probably take an hour or two to machine the component. The threading being the tricky part. Also, if you were to want a hex machined onto it, I would have to visit my mate at his 'engineering porn shop' & blag the use of one of his milling machines.
It's always better to have the spindle available, when making such things. As, threads being what they are. If I were to make a perfectly dimensioned internal thread using full form inserts, it probably would be too tight. Hence the need to match the item being made to the spindle it's going to be fitted too.
If it's a 2.5mm pitch one can use a tap to good effect,-bore item to tapping size, bung the tap through, Job done!
Be carefull if using stainless steel. A lot of stainless is pretty soft, & not suitable for an item that will have a high torque on it. I have a lump of (bloody expensive) 40mm diameter S143 grade stainless.This grade is typically used in the Aerospace industry, & would do the job nicely.
Give me a call if you would like a waffle on the subject.
Cheers.