As far as I know the main difference is that the early 16701A expansion frames were 2 fan models and the later 16701A expansion frames were 3 fan models, while all 16701B expansion frames were 3 fan models.
The 2 fan models have 2 fans on the side and none in the center strut. The 3 fan models have 1 fan on the side and 2 in the center strut.
I believe this change was required due to increased cooling requirements of newer analyzer modules, and possibly that is what this document is about, although it doesn't mention the details of the hardware upgrade:
http://literature.cdn.keysight.com/litweb/pdf/16715-92007.pdfFrames with the following serial numbers need to be upgraded to operate with 16715A, 16716A, 16717A, 16718A, 16719A, 16740A, 16741A, 16742A, 16750A/B, 16751A/B, 16752A/B, 16753A, 16754A, 16755A, 16756A, or 16760A measurement modules.
So while either a 16701A or 16701B expansion mainframe should be good with either a 16700A, 16702A, 16700B, or 16702B mainframe, some early 16701A expansion mainframes may not be supported with all analyzer modules. (Also some 16700A and 16702A mainframes may not be supported with all analyzer modules.) This may really be a case it works at your own risk even if not officially supported.