AT stands for "Assault tank" - these are the Nuffield designs, turretless, on the heavy side it seems. You can read about them in detail here. The source of this info is Yuri Pasholok via this site.
Tier 5 AT-2
A Nuffield design from 13.5.1943. 41 tons, torsion bar suspension. It was introduced along with the AT-1 design, which had a turret with a 75mm gun or a 95mm howitzer. With AT-2, it was decided an assault tank doesn't need a turret and so it was removed. Both shared the same hull, but AT-2 had a casemate construction, which saved like 4 tons of weight, compared to the AT-1.
Tier 7 AT-7
AT-7 is a weird project with a 6 pounder gun as its main armament. Basically, it was a re-worked version of the AT-6 (from May 1943), in which the developers decided to put a 6pdr gun in a sponson mount (and LOTS of machineguns in miniturrets). Later they decided it was a stupid idea anyway and they returned the 6 pounder (SS: in game this will most likely be upgraded, no was a 6pdr can suffice at tier 6) to the casemate construction, but didn't want to waste the perfectly good sponson (*facepalm*) and so they fitted it with a 20mm autocannon. Later (June 1943) the vehicle was reworked by changing the superstructure and the machinegun turrets. The whole thing had 43,5 tons in the end. There was a lighter 41,5 ton version of this project, designated AT-7A.
Tier 6 AT-8
This was a "sister" project of the AT-7 (AT-7: 10.6.1943, AT-8 29.5.1943) with the same 6pdr gun, but this time mounted in the center of the vehicle. The 20mm Polsten autocannon moved from the left sponson to the right sponson, the armor remained the same. The weight was 41 tons.
Tier 8 AT-15
The AT-15 is much like the AT-15A vehicle already in the game, but 5 tons lighter (60 tons). AT-15 comes from 21.9.1943, AT-15A is roughly two weeks newer. It was equipped with a 17pdr gun. It was a result of the army getting pissed at Nuffield, because Nuffield engineers loved to put the gun on the right or left side of the vehicle, but not in the center. The last such an assymetrical vehicle was the AT-14 project, after that it was reworked to mount the gun in the center and re-designated AT-15. The frontal armor is roughly similiar to the AT-15A: 152mm in the front, 101mm on the sides (respectively 6,5 and 4,5 inches)