Arguing about subtle distinctions in interpreting the practical applicability of features can be ... unproductive.
The original sear mode & operation of what Walther currently calls their Anti-Stress design was made to offer some advantages to a broad range of anticipated LE/Mil users at the international level. A simple reading of the Carl Walther safety manual (or even different releases of the European brochures) makes this rather apparent.
Walther even changed their trigger safety design over the years for some models, apparently in response to input from some international users. Not surprising.
I ignored the original P99's until S&W and Walther entered into their Strategic Alliance, and S&W decided to produced a licensed version of the 99 series for US sales. I signed up for what became my first of 3 P99/SW99 armorer classes. I was impressed enough to order a SW99.
As time passed, S&W and Walther eventually identified a magazine related problem with their .40's, and both S&W and Walther made some further revisions and improvements to their manufactured parts. I picked up another SW99 and continued to learn about the 99 series. I continued to support, maintain and repair some SW99/P99's in my capacity as an armorer.
While Walther introduced the idea of replaceable backstrap inserts as a common feature, I never found their original inserts to exactly fit my hand. Close enough to be acceptable, but other designs and models have evolved which offer a wider range of user fit and comfort, I think. Obviously,
subjective is the word of the day.
The 99's aren't something designed (or intended) to be tinkered with by owners. The guns can be damaged if improperly disassembled beyond basic field-stripping.
Walther doesn't even want its armorers to disassemble the sear housing blocks. Some basic instructions for re-installation of the single action sear lever and it's spring were included in some versions of the armorer manuals, in the event the plastic pin holding them in the block slipped loose when the blocks were removed for servicing, (or shipped as spare parts in baggies
).
Then there was a revision of the Walther frame which required a revision of the housing block body, as well, which further complicated sear housing block replacement in older models. Then, later they changed the trigger bar guide from a steel post (of different heights) to an adjustable lever, and that made it so the blocks of each type aren't interchangeable between the frames at all.
Personally, I think the 99 series, in all the model variations, have always been vastly under-rated and never received the appreciation they deserve. I prefer the AS model, myself, but then I'm a long time user of TDA (also called DA/SA) working/defensive guns.
I hope that at some point Carl Walther introduces a limited lifetime warranty to the original American owners of their excellent P99's, much as they did with their PPQ's
for American buyers.
The 99 series pistols are robust, reliable and nicely engineered.
I have no plans to get rid of my SW99's as long as I'm still able to shoot them.
Back to the OP's question ...
If the 99 being considered is a .40, then I'd make sure the magazines have the revision to help prevent lateral displacement of the bullet noses under recoil within the mag body (which might otherwise sometimes bump the slide stop lever tab and lock the slide back with rounds remaining in the mag). A visible change to the left side of the mag body, at the cutout for the slide stop lever tab, is easily seen.
This picture shows the S&W branded Mec-Gar .40 mags, with the
revised mag body on the left (and the revised follower, to accommodate the change in the mag body profile). Pics of the same revision for Walther branded P9940 mags can be found online (with their own change in the shade of blue for the revised followers).
FWIW, the PPQ version of the 99 has its own armorer manual, while the P99 AS / P99 AS C / P990 / P99 QA / P99 QA C / P99 DAO / P99 DAO C are covered in a different manual. (Last I saw, anyway, and these things can change without notice.)
I have no idea if the newly formed Walther Arms, Inc -
http://www.waltherarms.com/about-walther/ - will offer armorer training, or if they even plan to try to enter the LE/Gov market in the US, so I doubt I'll again be recertifying as an armorer on the 99 series in the future. Oddly enough, however, I was told a little while back that S&W did an armorer recert class for a special security agency still using the original SW99's, even though they no longer specifically list the SW99/P99 class in their field class curriculum.