This I know: 1st letter is the engine series, FJ was the original gas powered north american truck, but there was also the BJ (yep I laughed too) powered by a B series diesel this was in the 40 and 70 series too, also the HJ powered by a H series diesel in 60 series, HDJ (80 series) HZJ gas (80 series) PZJ (70 series) LJ (70 series prado)
FZJ (80 series) and so on the first letter or two corespond to the type of engine in stalled in the J chassis.
The J? I no Idea man. Like it was mentioned before possibly because they were WWII jeep copies or it could have been a left over from some military designation.
Yep I need a life