Very briefly, the first generation P99 handguns had the "hump" in the triggerwell and the short magazine release. They may be more difficult to find (new) now.
The AS trigger is a DA/SA trigger as any other, but the first SA trigger pull has a long, light trigger as opposed to the subsequent short light triggers like you'd expect. Decocking the handgun gives you the expected harder long trigger.
I don't have a QA, but it's similar to the Glock's precocked DA trigger as I understand.
Consider looking up the Walther Forum for detailed info on these handguns. Plenty of detailed info there.