factory suspension bags?

[screwballl]

ok i had a few free minutes looking over my new 91 burb R2500 and i noticed that it had what looked like a schrader (sp?) valve on the wheel side of the rear splashguard/cab step. I followed the line to see that it has bags between the axle and frame. This is in addition to the regular shocks and leaf springs.
I was wondering if this was a factory option or if it is aftermarket setup? I've never seen this before or setup like this. I tried to take a picture of the schrader valve but it came out too blurry and its too dark now but i did get a good picture of the bag on the passenger side...



Factory or aftermarket?
also does the bag look in good shape? i am curious to add some air to see if theyre still in working/good condition. Also if possible what is the psi rating of this type of setup?

Edit: hmmm found another pic and tried to get a better description as to the schrader valve location:

i appreciate any help possible.