You're right! I checked and the 610 to July '73 with L18s were PL610s.
My info shows the 710 as HL710 with all L18 motors.
In the 620s L16 were used till '73 and L18 to '74 and were both labeled PL620. L20Bs were the first HL620s
Yep, J stands for solid rear axle. And K is for coupe as usually. H is not necessarily associated with L18. Very often it stands for “the second biggest engine”, but not always. I haven´t figured out the logic behind this yet.
For example HLB310-> H=A14, L=LHD and B310 is body style. Again PLB310->P=A15 (the biggest engine available) and the rest is the same.
On the other hand, H is missing from the F10 series code. LF10->[blank]=A12, L=LHD and F10 is body style. Usually no engine code means the smallest engine, but this case the A10 powered version is BLF10. And like many other models (but not all of them), P stands for the biggest engine->A14 powered version is PLF10 (and coupe naturally is KPLF10).
Then there´s also bunch of letters after the model code. They define marketing area (Europe, USA etc.) and is Deluxe, SSS or what. But those letters are not visible in the code stamped on the car.
I would say there is a certain logic behind Nissan model codes, but at the same time it is highly illogical. I´m currently studying them. I´m writing an article about the codes together with couple of fellow Datsun enthusiasts. It will be published in the Nissan Club of Finland club magazine. Our research is still in very early state and I don´t know when the study is ready to bee published.
edit: zerow was faster than me...it is even more confusing if P is for L18 in US...I think those letters go after JDM models and since US models had "over sized engines" the logic iss gone?