James,
The new-style P99 frame has an accessory rail that looks like it will fit the lights/lasers such as the ITI M3 or M6 families, but it will not. It will fit the ITI M5 light. What's the difference? The position of the locking bar groove/notch in relation to the front of the trigger guard, specifically that the distance for the M5-style rail is 1/4 inches less. If you look at the trigger guard and the location of the forward locking bar groove/notch, there is little room to work with. The groove/notch is as far forward as it can safely go.
The P99 Compact's accessory rail is much, much shorter, and is designed to accommodate the tiny ITI XML light.