The M5 fits the P99 2nd generation .....
also Laser Devices Las/Tac2 tactical light
and there is one other one that is offered with a "S&W key" that will work also on the P99 but I forgot which one it was
Anything from 2004 on has the same rail. However - the inset portion under the frame - where the light locks into place - the P99 cannot use lights that fit on the Glock or other guns.
It must be a light made specifically for S&W semi autos. They do make a TRL1 with a S&W "key" - or, U can get the M5. The M5 is discontinued (it is just an M3 with the "key" in a different position to fit the P99 and S&W autos) - but there are tons of them new still out there at Internet vendors.