Smith & Wesson made the slide and barrel only on many of the P99's in 40 S&W with the OD Military frames. That is why they lack the German proof marks. The frames are made in Germany. I have sold, handled and fired many of these guns. While I may prefer a Walther to be all German made, Smith & Wesson has been around a long time and I would seriously doubt there is any difference in the quaility or specifications.
Others will say its not as good, but I would say, show me the proof and I'm willing to learn. I'm sure you will be very satisfied with this or any other P99 you could buy, because its a fantastic gun.