score:3
There's no (public) complete list of Greyhound's partner bus lines, as far as I know.
But PRC appears to be Vermont Transit (aka Premier Coach). The one daily schedule you mentioned leaves Hanover, NH at 2:50 pm, arriving in White River Junction at 3:05 pm.
This corresponds exactly to Vermont Transit's schedule.
Concord Coach Lines and Vermont Transit are well known around here, as they provide services to New Hampshire and Vermont towns that Greyhound skips over (as they like to focus on larger cities).
Since this was written, Greyhound has redesigned their website, and now tells you the name of the partner bus companies when you select an itinerary.