Insert D adds compatibility for the following devices to the current BookArc (2015 and later):
13-inch MacBook Pro (USB-C, 2016-2019), with or without Touch Bar
15-inch MacBook Pro (USB-C, 2016-2019), with or without Touch Bar

Insert D is not compatible with BookArc mod, BookArc for MacBook Air (pre-2015) 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).