The pre-AT AICS certainly has the “classic” one piece skin look instead of the modular skins on the AT AICS, but underneath the skins the early AICS and AT AICS are quite similar.
There were a couple revisions of the early AICS (pre-AT AICS chassis.)
Early ones had a very tight trigger pocket that typically needed machining for aftermarket triggers. AICS made near the end of the production run prior to the switch to the AT AICS had a larger trigger pocket, and the AT AICS and AX AICS had the larger trigger pockets right out of the box. (I’m sure the ATX AICS does too, but I haven’t messed with one.)
IIRC the earlier AICS were welded, later ones went to a pinned/screwed/bonded construction. No real difference to the end user as it’s all hidden under the plastic skins anyways.
It’s been many nears since I had an AICS chassis (I had a couple pre-AT non-folders and later swapped them out for AT folders but sold them quite a few years ago) but I believe the earlier ones also had a larger square of aluminum around the bolt handle with the skins trimmed further back to accommodate that extra aluminum. The older stocks sometime needed that extra aluminum relieved for bolt handle clearance on aftermarket actions. The AT AICS did away with the large hunk of aluminum around the bolt handle and has a more open design to accommodate a wider range of actions.
The pre-AT AICS had a higher "water line" around the action of both the aluminum and the plastic stock sides and may need relieving if your action has a large side bolt release. The AT AICS has a lower water line around the action to better clear side bolt releases without mods.
Also, sourcing replacement one piece skins for the pre-AT AICS is getting more and more difficult as they haven't been made for years and are used only now. If you think you might want to change colors or something down the road, the AT AICS skins are much easier to find. You can make the AT AICS skins fit on an old pre-AT AICS, but you’ll have to do a bit of trimming in places, mostly around the bolt handle cutout.
The AT also changed to flush cups for sling attachment while the pre-AT AICS used HK style hooks.
Whichever stock you get, consider adding one of SamB’s picatinny front bipod blocks to it. It’s way better than using a spigot mount adapter with a picatinny rail on the bottom of it, as those wiggle. Sam’s adapter block is much more stout feeling.
Finally... it's been quite a few years since I sold off my AICS, so feel free to correct me if I posted any bad info...