The SW99 frames, all frame parts and all of the smaller slide parts (meaning except for the bare slide and barrel) were produced and provided by Walther. Aside from some minor cosmetic and ergonomic changes requested for the frame by S&W, they were the same as were used for the P99's. Some of the minor changes requested by S&W were eventually incorporated in the Walther models, such as the accessory rail type, and the elimination of the prominent "hooked" part of the frame above the grip (to reduce pressure into the nerves in the web of the shooter's hand during recoil, we were told).
S&W engineers even provided the hi-speed imaging to Walther engineers that helped finally identify the early slide lock condition that occurred in both early P99 .40's and the SW9940's. It was eventually determined that a revision to the magazine body, at the slide stop lever tab cutout, and a revision to the follower, resolved the early slide lock issue. We were monitoring S&W's attempts to help identify this issue with some early SW9940's (as was reported by some SW9940 LE users, but which had also been reported by owners of the early P99 .40's), and I remember being told (by someone from the factory) of some of the minor revisions being requested and attempted by both S&W and Walther engineers at that time. Finally, Walther requested the magazine body & follower change to address the issue. As I recall, hi-speed imaging done by S&W revealed that it was the recoil forces allowing the 3rd or 4th round in the magazine stack to wiggle laterally as the rounds were rising, under recoil, that was bumping the slide stop lever's inner tab.
Walther also eventually designed an interesting magazine safety for the Walther frames they were offering for the S&W licensed 99's. It was some time after my 3rd armorer class had occurred, but I'd heard about it during the development stage, when S&W was negotiating with Walther to design one for possible LE agency sales. I never got a chance to handle one in person, but I've seen a couple pictures of one that was finally shipped to the US and sold.
S&W never "botched" anything in their licensed 99's.
They actually made some nice revisions, such as through-hardening/heat treating their entire slides and barrels (versus the zone hardening done in the Walther slides and barrels, as that was reportedly all that was required in order to meet the older Proof House requirements in Europe). They revised the chamber mouth design at least a couple times (chamber support, feedramp and roll-over notch) compared to the original specs based on the Walther barrels, and revised the bottom of the barrel to incorporate a machined relief cut for the rear of the RSA to better clear the barrel (that noisy scrunchy sound, as the barrel scraped over the edges of the spring coils). I saw a couple of versions of that revision come through in new barrels, as the engineers were apparently refining the length of the relief cut.
Another nice revision adopted by S&W was to change over to using the "optional" heavier extractor spring in most of their 99/990L models. This was actually the same spring used in the magazine catch, and was later listed used in some Walther pistols as the new standard spring, if i remember a Walther parts list I saw some years ago. S&W engineers also revised the orientation of the extractor spring, reversing the how it was inserted into the slide's spring hole. We were told that this reduced the chance for an incorrectly seated spring (the large end was a tight/press fit in the hole), and the smaller end being inserted made for a more flexible fit and function.
I saw some refinement in the way Walther extractors were radiused and polished over time, and how they machined their locking blocks, too.
S&W requested a longer and stronger tension slide stop lever wire spring, but it had an "open" hooked end, versus the shorter closed/loop design of the Walther spring. S&W finally provided some of the Walther designed springs when it was reported that some issued users were inattentive during cleaning practices and were stuffing towels/rags into the mag well, which hooked on the wire springs and bent them. The closed/loop design might not have been as strongly tensioned, but it was less susceptible to that user-induced damage.
Granted, S&W did do something a bit odd with their 990L models, which were their licensed version of the Walther Quick-Action (QA) model. The Walther QA incorporated a small decocking "field stripping" button in the slide, to decock the striker firing pin and allow for field-stripping. For whatever weird reason, S&W decided to eliminate the small button in their 990L slides, and have the user/owner press the trigger to decock the pistol for field-stripping. When I asked in a class once, I was told that the consensus was that corporate had seemingly decided that keeping the field-stripping manner similar to their other plastic model (the Sigma) was likely less confusing. Dunno, but that was the conjecture.
S&W apparently didn't produce many (if any) of the early 990, which was the original DAO model of the 99 series (later called by Walther the P99DAO), although they did agree to make that odd SW99NJ model, which earned it's own spot in one of the SW99 armorer manuals. There's a lot of rumor about that short run of guns made for the NJSP, but the long and short of it (according to what we were told by factory sources) is that depsite some political machinations of the time, that independent testing performed by HP White Labs revealed none of the reported problems in extended testing, and the guns were later sold off by S&W through their distributor network. As I recall, the mean rounds between failure observed by HP White Labs was very respectable for any duty pistols of that time (as was the in-house factory testing).
Now, Walther was busy making its own ongoing refinements to their models, many of which weren't easily seen by the naked eye. The design of the sear housing block required a revision to the inner dimensions of the frame, and the change from a trigger guide post to an adjustable lever also required a frame dimension change. (The difference in the sear housing block guides means they are
not interchangeable between those 2 revisions, while an earlier revision of the shape of the bottom of the SHB merely required replacing one tube pin in the block with the previous one when replacing a SHB, due to a dimension change that resulted in a change to the length of the pin.) Then, of course, Walther introduced their ambi slide stop lever design, an additional trigger option, the PPQ, etc.
The 99 series is an outstanding design, and let's not forget that it introduced the idea of the replaceable backstraps. It never really received the attention in the US that it gathered internationally, though. Talking with a couple of the guys at the former Walther America business that was operated by S&W (during the 13 years of the Strategic Alliance between Walther and S&W), and within the S&W factory for the SW99/990L production, it wasn't uncommon to hear that Walther didn't seem overly interested in having S&W do much in the way of heavily advertising their P99's. Presumably, now that Walther had gone to the effort of creating their very own import company and headquarters in the US, that's been changing.
It's also nice (for owners) that Walther has finally adopted a limited lifetime warranty program for their firearms, versus the previous 1-year warranty they offered. (The PPQ was the first model line to get that warranty "upgrade", and that was listed as only being for American buyers of the older PPQ models.) I remember referring one P99 owner (from the forums) to S&W because of a P99 slide that snapped part at the rear edge of the rear sight dovetail cut. As I remember, S&W was told that Walther would not warranty the broken P99 slide because it was a few years old, so the S&W customer service people found an "extra" new slide from a P99 Titanium model in their repair dept and offered it to the owner, at no charge. They ate the cost of the slide in the interest of Customer Relations, even though it was a Walther product that wasn't considered to be under warranty any longer.
