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you thread left-to-right (away from the chuck) for left-handed threads
You can do it for RH threads too by reversing the spindle and inverting the tool. It's not a bad way to do any thread which is open at the tailstock end, but for the part in question I'd consider it pretty much essential on a manual lathe, since you can't afford to lose thread length by creating a decent relief against the shoulder.
In case you muff up the stops? I don't think I ever cut threads on a lathe - does the tool have a tendency to keep going?