I believe we all agree on "field" and before that seems to be either an "n" or "m"
go here:
https://www.postalhistory.com/postoffices.aspType nfield or mfield in the search box tick "string search" and most every post office with the letters nfield or mfield will appear sorted by state and with it's county listing and year or years of operation.
It is a great resource created and offered for free by Jim Forte.
Oh, and what Richard Frajola said should not be ignored.