The original SW99's had the Walther AS trigger (the sear housing block controls the 'action'), so they were traditional double action pistols and equipped with the Walther decocker button.
The SW99NJ was an interesting variation which S&W reportedly didn't really want to make the way the agency wanted it made, and there's seemingly more to the story which isn't really relevant to this thread. FWIW, the second SW99/P99 armorer class I attended had a manual which listed the SW99NJ. I'm keeping that manual as a potential collector's piece.
Anyway, the SW990L, or the "Lightning" version as it was being called by some folks at S&W when I first heard about it, features the Walther QA trigger. There's an interesting difference in the SW990L pistol compared to the Walther P99 QA, though, and that is that the 990L doesn't incorporate the reduced profile decocker button in the slide that Walther uses in their QA models. Instead, S&W decided to use the 'smooth' DAO slide (think of when the early SW99 manuals first listed the SW990, which would've been similar in operation to the original Walther P990, which was
really DAO, and therefore didn't have, or need, a decocker).
That meant that in order to release the striker of the partially cocked 990L the user has to pull the trigger on an empty chamber, in the same manner as on the Sigma (and the Glock). On the Walther P99 QA the user depresses the reduced profile decocker in order to release the partially cocked striker.
The Walther P99 AS and the P99 QA models use different striker springs, too, with the QA model having a slightly heavier striker spring.
The trigger bar guide, a small steel part located on the bottom front of the sear housing block, can be different, as well.