I think all P90's should come with that built in laser sight in the handguard, screw the laser on the side rail.
Well anyways Tem, alittle food for thought, the Tri Rail P90's are the most widely avaliable version of the P90 compared to the RDS P90's. This is just because the built in RDS on most P90's aren't always the greatest quality, now TM did make a newer model of the gun with an updated RDS which actually works, however I have not seen one other then DarkAngels on ASC.
Now since you maybe getting that aftermarket tri rail though, I guess the quality of the RDS won't matter as you will replacing it
Ohya and for that tri rail you originally posted, like Kos-mos said, it can ONLY be used a RDS P90 receiver.
Enjoy