With regard to the particular conversion here, Eitel (p.

11) says the Naga's name was Apatala, the guardian deity of the Subhavastu river, and that he was converted by Sakyamuni shortly before the death of the latter.
(4) In Chinese Na-k'eeh, an ancient kingdom and city on the southern bank of the Cabul river, about thirty miles west of Jellalabad.
(5) We would seem now to be in 403.
(6) Soo-ho-to has not been clearly identified.

Beal says that later Buddhist writers include it in Udyana.

It must have been between the Indus and the Swat.

I suppose it was what we now call Swastene..