First, here's the 999/X info, in German, no less (click to enlarge):
View attachment 3827852
Compared to the specifications for the
body of your 999/X, the 90 EE/X, the 909/X (
NOT the 909/X Improved!), the 909 E/X, the 999 E/X, the 999 PE/X, the 999 SE/X, and the 999 TE/X are exactly the same. The 999 VE/X and 1000 ZE/X bodies differ only very slightly, as Free Speaker said. In other words, the cartridge model is determined solely by which stylus is fitted. Think of it as one cartridge body, many styli; therefore many "models".
I suggest trying your 1000 ZE/X stylus in that 999/X body to see if the pops are still present. If they are, it could be one of a handful or reasons: static buildup and discharge, damaged rubber suspension, loose fit of the stylus in the cartridge body, et cetera, and I hope someone who's had the same problem will provide you with some helpful information!
Here's the entire 1971 model lineup:
View attachment 3827866