The D in nissan designated engines is for dual over head cam which it is not.
the designation is as followed:
S = Sport model
L = luxury model (I don't know why, they are all sports car)
RB = Race Built
## (i.e. 24) engine displacement
D = dual overhead cam
E = electronic controlled engine management
T = turbo
R = race
So a 240z's engine would is called L24 and a 280zx's engine would be L28et
and in the case of the fairlady z432 and z432r (JDM only) it would be RB20d, and I don't think they made a turbo version for the z432® models. The RB20d was originally called the S20,but was changed when they started putting them in the next line of cars like in the Skyline r31 (version 2) and in the JDM version of the Maximas (Cefiro in japan). The early Maximas (Cefiro) originally had a SOHC thus being RB20e. If any of this is wrong, have any contradictions, or if you would like to add something, please do reply.