Alot of ranks here in NW are based off of modern ranks, example the French rank Soldat de Premiere Classe, Private First Class didn't exist back then for all i know, but today it is a distinction in the French army.
If you look at a french company it will tell you: 1 Captain, 2 Lieutenants, 4 Sergents, 8 Caporals and the 125-ish Privates. If you were below a corporal you were a private and there was nothing below private.
As for the rank ' Kingsman ' i have never come across it reading about the British army in this period ( Even though i don't read much about them ). I'm guessing sometime long ago somebody found the rank on a random internet site which talked about ranks of the present day (i.e. Duke of Lancaster Regiment ) and decided to add it to his ranks for his regiment to create a chain of command and to give rankers chances to rank up without instantly becoming an NCO.