The thread would have been created by sticking a prong through the vessel while it was still hot; perhaps two prongs, meeting in the middle? Similar to the way "ice-pick" flower bubbles ae created in paperweights.
The maker may have had to pull the prong right the way through. I'm not exactly sure how it was achieved, we might need a glassmaker to come along and tell us exactly how.
I don't know whether or not they'd have done the thread or the base first.
