I'd guess F9B-1 and F9B-2 - the ninth fighter from Boeing with the "E" being the dash 1 and the "F" the dash 2.
Or, they might have choosen to resurrect the old BF (bomber/fighter) designation which was used between 1934 and '37 - in which case, the Super Hornets would be the BF2B-1 and the BF2B-2.
{The XF8B-1 was the previous last Navy fighter built by Boeing while the
1935 Boeing XF6B prototype was redesignated as the XBFB-1. Thus "9" would be next in the Boeing fighter sequence, while "2" would be next in the Boeing bomber/fighter sequence)Cheers,