I'm not sure about the differences but I know what the AS is. The AS stands for anti-stress trigger. When it's cocked, the trigger doesn't change position until the slack is taken up. It will click at the reset and stay there. It only works this way for the first pull of the trigger. Every other pull is set at the reset. It also has the decocker to put it double action. It's a little was a little different at first since the only handgun I had before was an Springfield Amory XD.
Wait, this will explain the AS trigger system better than I can
From Walther's German website
Traditional double action and single action modes, and the added feature of the anti-stress trigger position which is engaged after reloading.
The P22 was the most recent purchase. I bought the P99AS from Gander Mountain for $519.99 + tax a couple weeks ago. I ended up getting four magazines with the gun. Two 10 rounders and two 15 rounders. When I bought it, the display I looked at had 15 rounders. The one I took home came with 10 rounders. I went back to complain and they ordered the 15 rounders for free ;a