Jason’s answer is correct for the dozer callsigns with one per Squadron in the SHQ. One per troop would have been really nice but it wasn’t to be.
In your picture, the callsign 13B denotes the third vehicle (B) of the third troop (3) in A Sqn (1). You won’t find a Delta callsign in a tank troop. You will find one as the junior callsign in the second patrol (2 vehicles) of a recce troop (M113 C&R Lynx) - typically a troop leader plus 3 two-car patrols. In higher formations (Brigade, Division, etc) a recce troop could be larger. Hope that made sense.
I’ve been following build and think it looks great. With the effort you’ve put into it, you’re going to end up with a nice looking model. I’m looking forward to your posts.