The NPL was a new competiton at the time (1968), formed as a northern equivalent of the well established Southern League. Peter Swales, incidentally, was a key figure in making it happen.
It wasn’t a merger or a rename of any existing league. Rather, its original member clubs were selected through an application process, with clubs from five existing leagues - Cheshire, Lancs Comb, Northern Regional, Midland and West Midlands (Regional) - chosen to be the founders.
The Cheshire League and Lancs Comb carried on through the 70s and early 80s before merging with each other to form the NW Counties League. I don’t know how these two leagues compared through the 70s once they’d each lost their best clubs to the NPL but, by the time of the merger, the Cheshire League was certainly the stronger of the two.