I've never heard of two types of mags for the P99 .40, much less one with "issues" and another one that doesn't have them, and I don't think that dimples are going to show you anything.
The early P99 .40 mags did have issues. The easiest way to tell if you have an early mag is the color of the follower.
The original P99 .40 mags with the orange follower had issues where the baseplate could break pretty easily, and have the spring, follower, and all remaining rounds in the mag fall out of the bottom of the pistol while the mag body was still latched onto the frame of the pistol.
Mec-Gar revised the magazine and changed the color of the follower to dark blue to differentiate the mag from the previous design.
Then, it was found that the newer mag also had an issue where the rounds in the magazine could shift under recoil, and the bullet nose could hit the tab on the slide lock lever as it was moving up the mag body, and cause the slide to lock back while there were still rounds in the mag.
Mec-Gar revised the magazine design again, and this time they changed the color of the follower to light blue. These light blue mags, and all mags made afterwards seemed to function very well. I haven't heard of a mag issue with the .40 mags since.
Mec-Gar's change to the light blue mag design happened in the early 2000's, so I'm not surprised if people never heard of these issues, but the early .40 mags did have issues. Back then, Walther would replace any older mags that were sent in, to the newer design.
I don't have any pictures, and I sold my .40 P99 more than a decade ago, but it should be very rare to find one of the earlier "orange follower" or "dark blue follower" mags for the P99. Any P99 .40 mags that do not have an orange or dark blue follower, should be good to go.