Insert B adds compatibility for the following devices to the current BookArc (2015 and later):
11-inch MacBook Air (2010 - 2017)
13-inch MacBook Air (2010 - 2017)
13-inch MacBook Pro (Retina, 2012 -2015)
15-inch MacBook Pro (Retina, 2012 -2015)

Insert B is not compatible with BookArc mod, BookArc for MacBook Air or the previous version of BookArc for MacBook (pre-2015).

Not sure which BookArc you have? Previous versions came with numbered inserts (1-8) while the current BookArc has lettered inserts (A-H).