The Mec-Gar/Walther 16 round "unmarked" magazines for the 9mmx19 P99 are, according to Earl Sheehan of Earl's Repair Service, Inc., actually for the Walther "HLP" (hammer-less psitol) which was the original designation of the P99 and they were made prior to 9/13/94.
Walther and Mec-Gar both claim that NO pre ban, non-LEO/Restricted magazines were ever imported into the US as complete magazines or, magazine bodies. Walther-USA, now Walther America says "not available or never were" during a phone conversation, except for the "restricted" mags for those who can legally purchase and own them.
Karl-Heinz Luther of Walther Germany says, "we assume that provate individuals purchased the magazines in Europe and brought them to the USA".
This also includes the 12 round .40 S&W P99 magazines.
The ATF opinion on this is that they are illegal and they cite USC section 921(a)(31) of Title 18.
See my web site for their correspondence direct to me via e-mail.
but there are no date markings or serial numbers to verify these "facts."
I'm betting that date of manufacturer could be established by tracking the patent number on all these magazines.