So This Rn-X7 Has Me Stumped

I purchased a big revenue collection that had several dozen of these RN-X7s. No big deal (most were from Pope Manufacturing in Hartford CT, which is cool since I'm a cyclist though), but three of them have a mysterious "N" within the vignette. One is from March 1900 (#142), one from June 30 1900 (#163) and one form Sept 29 1900 (#177). So these three aren't sequential. (Many of the ones I have are). The Ns all look to be pretty consistent in position relative to the vignette. It's not printed on the back, but it appears they might be under the vignette. Has anyone seen anything like this before?

It might be "N" from North or New England.
There is an article referring to 'National Bank Notes Collecting' and half-way through it is mentioning 'Geographic Letters':
http://www.numismaster.com/ta/numis...icleId=16397