There is no such thing as a "2nd Generation" P99 frame. The seller's claim is dubious. It certainly looks like a standard Walther P22 Laser, which WILL slide on a P99 Compact (which I believe is the same as the Post 2004 Full size P99 - but I cannot verify this). It will NOT lock in place. There has only been TWO styles of frames and rails for the P99. Pre-2004 (proprietary) and Post-2004 (open ended S&W Rail compatible with SW99 and "Tactical" rails). There may very well be a "New" P99 laser for P99 Post-2004 frames, but I'm not aware of it and because of the seller's other bogus statements - buyer beware.
The pictures shown above show a P99 Pre-2004 frame (1st Generation), a P22 Frame (2nd Generation - actually P22 ONLY but I guess it did come out after the P99 Pre-2004 and before the P99 Post-2004, so technically you COULD call it "2nd Gen") and a Post-2004 P99 (3rd Generation - actually 2nd Generation of the P99 frame). Then there is the P99 Compact rail. Now, I don't have a Post-2004 Frame, but I believe the P99 Compact and Post-2004 P99's share the same rail configuration.
Accessories and what they fit on:
P99 Laser (Original BA-2 and BA-6 Walther Lasers) - Gen 1 Frame ONLY!
P22 Laser (P22 ONLY!)
Springfield/Insight XML/X2/X2L Light/Laser - P99 Compact (May fit Post-2004 Frame). DOES NOT FIT P22!
Insight M5 Light (Fit's Post-2004 P99s)
Insight M3/M6 Lights DO NOT fit Post-2004 Rails without the M5 adapter from GG&G.
Laser Devices has new accessories for the P99 2nd Gen Frame.
Link to Laser Devices Walther Accessories
Regards,
James