It would be the P99AS...
What other magazine from the Walther pistols is compatible with it?
Magazines for the Walther PPQ, Smith & Wessoon SW99 (a P99 clone), and Magnum Research MR Eagle (another P99 clone), are all compatible with the P99.
All of these magazines are identical (aside from the names on the baseplate), and are made in Italy by Mec-Gar. They are all of excellent quality. The Magnum Research MR Eagle mags tend to retail for the least (~$32 each).
Do not, under any circumstace, buy the Pro-Mag P99 magazines. They are trash.