OK, Bought a 07 with a couple problems today.

[SpareParts]

Think I have the 6JL and 7JL code's figured out. It's for the springs.
6, 7 are left and right. Some places list it as 6 left and 7 right. Other places list it as 6 or 7 meaning either side.
Looking at pictures i do not think springs are wound left hand and right hand and can go on either side.
JL, is the code for the spring itself. Different codes denote what the spring is used for like 1500, 5.3, 18in wheels few options would need a lighter spring rate then a 1500, 6.2, 20in wheels full option.
There are about 20 different codes for different springs and there is no way i can figure out each one.

 

[Fless]

I haven't read all of this thread but is there a chance there are labels with part numbers still on the OE springs?

 

[SpareParts]

Maybe, I have not looked as im probably putting Moog HD springs on it.

 

[petethepug]

GM did a really good job of camouflaging their spring data. Factory installed springs are spec’d for L & R and they later contradict themselves and spec them for either side. You can also use the paint on them to differentiate. Sometimes the tags are still legible.

RockAuto has a pretty good list attached to the z55 shock selection of the RPO codes associated with them. Lately I’ve been seeing more and more GM front springs for z55 pop up on R/A & parts geek.

 

[SpareParts]

RA does not list a spring for GM for 6JL/7JL.
Im going Moog HD springs for all 4 and then the RC 3.5 in kit. Mostly for the upper control arms but it has most of what i need.