We develop apps for several US states and I am putting together a demo version. I want to use the (currently) fictional state of Franklin since it has historical legitimacy but I ran into a small problem: what is the two letter abbreviation? FR = France, FK = not safe, FN just does not convey meaning. Is there / Was there an official abbreviation for this state that never quite was?
The "Official" abbreviations that you refer to were developed rather recently. The standard two-characters-per-state was not created until 1963, and took a while before it was widely used.
Source: http://about.usps.com/who-we-are/postal-history/state-abbreviations.pdf
TLDR: No, there was no official abbreviation for Franklin.