S&W website downloadable manual for their various SW99 models (.pdf format).
http://www.smith-wesson.com/wcsstore/SmWesson/upload/other/SW99.pdf
BTW, there was an early revision of the .40 magazine bodies and followers designed to resolve the early slide stop engagement issue which was reported among early production P99's & SW99's.
The magazines can be identified by looking at the left side of the mag body, at the top, where the slide stop lever's tab reaches inside the mag body. The revised .40 mag bodies have a flat spot pressed into the side, to the rear of the lever's cutout, designed to help prevent the cartridge bullet noses from shifting leftward under recoil and bumping against the tab, lifting it upward to engage the slide while ammunition was still remaining in the mag.
In this image the original mag is shown on the left, with the revised mag on the right. The follower had to be revised, as well.
S&W made some further changes to their barrel design, making some subtle changes to the feedramp, chamber mouth and muzzle crown, as well as incorporating a flat area on the bottom of the barrel (the exterior) designed to reduce contact between the rear of the recoil spring guide assembly and the bottom of the barrel during unlocking/locking (cycling). Walther also made some slight changes in the frame they produced for the SW99 series, as well as some of the Walther frame & slide parts, but only an armorer would really need to be aware of them if a sear housing block were being replaced.
I carried a SW99 .40 for a few years as an issued weapon. I own one of my own, too. Decent gun. Typical snap and muzzle whip of a lightweight, plastic .40, though.
The rear sight and front sights have some minor issues to consider when carrying the gun (or replacing the front sight post), but nothing critical (unless you plan on knocking the rear sight base against a hard surface, which could damage the read sight base plunger).
Although I still occasionally shoot a SW99 .40 for practice, I've focused most of my attention on my SW999c (9mm compact). I tend to really like the 99 (Walther or S&W) series when it's chambered in 9mm.