What does the last 'B' in 'GEnx-2B67B' stand for?
Based on the below (in chronological order), it very much looks like the (or a) difference between GEnx-2B67 and GEnx-2B67B is a booster anti-ice (BAI) air duct. And this suggests that the 'B' stands for 'BAI' or 'booster anti-ice'. 


  We propose to adopt a new airworthiness directive (AD) for all General Electric Company (GE) model GEnx-2B67 and GEnx-2B67B turbofan engines with booster anti-ice (BAI) air duct, part number (P/N) 2469M32G01, and support bracket, P/N 2469M46G01, installed.


https://www.federalregister.gov/documents/2013/04/11/2013-08447/airworthiness-directives-general-electric-company-turbofan-engines


  The Boeing Company and GE asked that the GEnx-2B67 turbofan engine be removed from the Applicability section of this AD. The commenters noted that this engine does not have BAI hardware, therefore, inspection and modification of the BAI does not apply.


https://www.federalregister.gov/documents/2013/08/19/2013-20097/airworthiness-directives-general-electric-company-turbofan-engines

NB: Another hypothesis, that the 'B' is some kind of alphabetic numbering, seems refuted by the lack of an 'A' or 'C' model. Google will find very, very few (1 or 2) hits for 'D' and 'E', which could be typos.