Things do line up internally differently between the QA and AS.
Yeah, they have different sear housing blocks, being different 'actions', QA vs. TDA(AS).
I still wouldn't start mixing & matching components between pistols.
Last time I asked, I was told that S&W receives their 99/990L frame, frame components (except for the slide stop lever spring itself), the striker assembly, sights, extractor & extractor springs from Walther.
S&W makes the slides and barrels for the S&W licensed versions.
Dunno about the recoil spring assemblies.
Magazines come from Mec-Gar, but made to S&W specifications. I was previously told that this meant S&W engineers had requested differently rated magazine springs than were used in some magazines used in the Walther line. I was specifically told this meant they did not recommend that magazines be used interchangeably between each manufacturer's model pistols when it came to pistols dedicated to defensive usage.
The magazine catch spring is also used as an optional extractor spring in some models (being heavier than the 'standard' extractor spring), for corrective action/repair, and is the 'standard' extractor spring in some other models in the S&W models.
Now, there are also differently dimensioned trigger bar guides used in the 99 series (Walther-supplied part, being part of the Walther-supplied sear housing blocks). Look at part #43 for the MSW999C & MSW990L9C pistols in the factory parts list (toward the end of the list):
http://www.smith-wesson.com/wcsstore/SmWesson/upload/other/PistolPartsList_Retail.pdf
Although it's been a couple of years since I took my last SW99/P99 recert class, I was told that Walther has finally provided S&W with the technical specifications on all the various parts used in the 99 series, and that there's a tolerance (dimensions) range of some parts. If I recall right, there are actually a range of 10 different trigger bar guides which may be needed to make any given 99 model function as intended, even though most of the pistols will function fine with the 'standard' one for a particular caliber/model. If not, a particular pistol may sometimes have to be checked at the factory to find the right part needed to function as intended in that pistol. I was told by someone at the Walther America facility that a few P99's have required corrective action because of the trigger bar guide, and/or trigger bar assembly, which came with the pistol. I had that happen with a SW999C model, myself.
Sometimes parts which are combined and provide for proper functioning in one pistol may not come together and do so in another pistol ... which is not uncommon when it comes to pistols which use 'drop-in', non-fitted parts, you know. Sometimes parts is parts ... and sometimes you need a different part, or combination of parts, to make things run right in a given pistol.
If I had a couple of pistols which were similar ... like a P99 & SW99 ... I'd keep them in 'as-produced condition' and wouldn't start mixing & matching parts back and forth.
If one of them exhibited a functioning problem that required corrective action by parts replacement, I'd install the appropriate new part and check it for fit & function. If I couldn't get things to run right with parts replacement I'd call the factory and ask for advice, which might involve returning the pistol to the factory for them to examine and repair.
Sometimes a new part which might not work in one pistol might work in another one, though ... but it would still require armorer inspection for normal fit & function, of course.
Just my thoughts.
Congrats on owning a couple of fine examples of the 99 series, BTW.
My SW999C has become one of my most enjoyable training/practice compact pistols for the range.
