Shoku Nihongi
The was an old trade route in the Kiso Valley that stretched from Niekawa-juku in Nagano Prefecture to Magome-juku in Gifu Prefecture. There were eleven resting spots along the route, all of which became part of the Nakasendō when it was established. There is an article dating from 713 in the "Shoku Nihongi" that records the routes characters as 吉蘇路.