FWIW, this is what I received on the subject from Kurt Hindle, the head of Walther-USA:
I received your email, and I spoke with the gentleman (Larry) while you were on hold. Let me start by telling you that the P99 was developed, and first sold into commerce in 1998 (four years after the Crime Bill). The Walther P88, is a 9 mm double stacked pistol, which first went into production in 1985, and was a 15 round capacity magazine. I believe what you are seeing advertised as a 15 round P99 magazine, is really a P88 magazine. The magazines were I believe both manufactured by Mec-Gar, but their compatibility between one gun and another is anyone's guess. The P99 is 16 rounds. It was not manufactured, even in prototype form, before the signing of the Crime Bill in 1994. This means that there is not a "Pre-Crime Bill" hi capacity magazine specifically for the P99.
I appreciate him responding so quickly.