here are the 2 relevant drawings. I forget where I got them, I think from the former Yahoo groups.
The M19x1 threads I made M20x1 because by the time you are done making those threads you are less than 20mm diameter anyway, allowing the grinding wheel to slilde over.
Make a bunch, once you work out the production plan it doesn't take a lot to make 5 or more in sequence.
Gerrit