The SW99 line can be a little confusing.
If you look at the SW99 Manual ...
http://www.smith-wesson.com/wc...pload/other/SW99.pdf ... you can see where S&W originally planned on producing their licensed version with models similar to the Walther P99 series, meaning a SW99 (TDA - Traditional Double Action w/decocker, what Walther now calls their Anti-Stress, or AS); a SW990 (actual Double Action Only model similar to the P990); and the SW99 QA (Quick Action w/reduced sized decocker button).
S&W produced a limited run of their SW99's for the NJSP, named the SW99NJ, with the Walther TDA/AS sear housing block but without a decocker button (the 990 DAO slide). There's a lot of weird info floating around the internet about this model, but that's another story. I still have the SW99/SW99NJ Armorer Manual I received at a SW99/P99 armorer recert class which I'll keep as a keepsake and for any potential collector's value.
Anyway, S&W later started to produce a model they called the SW990L. Just before its release it was being called the SW990 Lightning by at least a few folks from the company.
Anyway, it was essentially a QA-type pistol but with a 990 DAO slide, meaning no decocker button. This S&W model differed from the Walther P99 QA model in the respect that the Walther had a reduced size decocker button on the slide and their pistol could be decocked without having to pull the trigger. The SW990L had to be decocked like the Sigma, meaning that the trigger had to be pulled on an empty chamber in order to field strip the pistol. I never got a definitive answer from the various folks who I asked about why this was done, but a common opinion was that it made it possible to treat the SW990L (and SW990) like the Sigma when it came to field stripping. Consistency in manipulation across a couple of their model lines, so to speak. Maybe so.
S&W has always been known to make limited numbers of various guns for their own reasons. Sometimes they weren't a regularly cataloged item and preceded a similar production model, or sometimes they've been prototypes for possible future production. (For example, the limited run of Sigmas made in .357SIG, the MSW357V, are popular with collectors.) They've also been known to occasionally sell off these pre-production, prototype or just limited production guns from time to time to their distributors and stocking dealers, which makes them very popular with S&W collectors.
Colt is like this, too. One of the standing jokes about weird Colt models is to never say they've never built some weird model, because some engineer may have done it just to do it ...
Now, I've never asked how many of the SW990 & SW99 QA guns they may have built. Although the models were (and are) listed in the owner Safety & Instruction Manual, they were only mentioned in passing in the 3 armorer classes I attended (which I attended before the 990L models were put into production). In the classes we were left with the impression that S&W would make the 990's & QA's for an order, like a LE contract or large distributor order, but hadn't decided when (or whether) they were going to start producing them. Dunno. I never inquired further about them.
There are some differences between the TDA (AS) and QA models. If I remember right the QA model has a heavier striker spring and a different trigger bar guide, for example.
You could always call back to S&W and ask to talk to someone familiar with the SW99 series, making mention of the specific SW99 QA model you've come across and asking for some info about it. Since the SW99 series isn't in regular commercial production anymore the folks that used to make them are probably assigned to other things, so it might not be a question anybody could answer just off the top of their head.
I'm certainly not an expert on these things, but I followed the SW99/P99 line for a while as an armorer supporting a number of them and as an owner of a couple of them.
Let us know what you find out, though.