I don't think the shocks or springs have anything to do with it has to do with how the suspension is designed. That's what I noticed when I took measurements before I installed Coilovers.

per material cost i am pretty certain both sides are even on stock setup.

for coilover setups you'd sit in the car while it's being corner weighted so that the car sits flat with you in the car.
same with alignments for racing setups.

If the car was designed to sit higher on one side than the other without anyone in it, that means the two front springs would have a different spring rate and coil compression.

Regardless, our stock springs make the car sit perfectly even from the factory all around. Likewise, my Swifts seem to have settled and are even all around since I broke them in correctly.

When I inspected where the rear springmounts attach to the bottom portion of the z I noticed that the left and right areas weren't mirror images. The two areas were different. I would suspect that would be what could create the difference in height. The difference isn't really noticeable by looking. The front were a couple mm different and the rear were several mm.

so if we get a right hand drive Z's service manual and compare it with a left hand drive Z's service manual we will know of sure if these parts are swapped in the two markets...

I was just guessing that the difference of the two areas was the reason, only because the two sides are different. But that wouldn't explain why the front left and right are a different height. I recall reading an interview of one of the Japanese engineers and he mentioned that the z was designed with the the u.s. consumers. He commented that the u.s. spec has better left and right weight distribution. Although I don't know think that he was talking about the left and right height difference.