The Missouri Code of State Regulations Information Bulletin governing modular units, Chapter 123 4CCSR 240-123. Effective March 30, 2018 all new modular unit models are required to be constructed to the 2015 IRC, 2015, IBC and the 2014 NEC. Approved existing models are valid until October 1, 2018. The Seal Application, Plan Approval & Plan Review forms indicating 2015 codes (as referenced in the letter) can be found in the link Here.
Full bulletin release Missouri Code of State Regulations Information Bulletin 05-10-2018.