Are the letters D and B just vehicle identifying purposes, as were numbers? So you can have quick and easy communication between vehicles and command.
Exactly, this was the classic artillery style marking for A, B, C, D... guns.
From late 1942 LwFD were equipped with four StuGs each division, that is until late 1943 take-over by the Heer and the number of vehicles being bumped up for Lw-PzJg-Abt's