I bought a FNP 45 tac when they first came out (NOT FNX). I just used the night sites that where on the pistol. Now I would like to mount a red dot. Do I just remove the plate cover that is on it and mount the red dot to the frame? Or I do I need a mounting plate of some kind? Thanks for any help you can give. I am thinking of this red dot below.

HOLOSUN HS507K-X2 COMPACT REFLEX SIGHT - RED CIRCLE
Red Dot Sight

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Available for In-Store Purchase or Curbside Pickup Auburn, WAShips from Distribution Facility
Holosun HS507K-X2 Compact Reflex Sight - Red Circle
$289.99

I have an FNX and it came with plates for various red dot types at the time. I'd imagine the FNP would be the same. Mine came in a plastic box in the bag with the gun. So yes you'll need a plate if that red dot won't work on the plates that came with the gun.

Thank you

You need the mount plate; it will have the recoil posts built in, has the proper footprint to seal the bottom of the RMR, and blocks off any through features that are cut into the slide for other optics.

Midwest Gun Works sells them.

ETA: I am reasonably certain the optic mounting interface was not one of the changes made with the update to the FNX from the FNP.

They are the same.  I have an FNP and FNX; plates are interchangeable.  If the OP doesn't have the mounting plates, you can order them directly from FN.  For the most part, the slides didn't change on the transition.  For the OP, not the 507k is the smaller 507 and DOES NOT have the same footprint as the 507C X2 (which fits the RMR plate).  I made the same mistake and ended up with a 507k which turned into purchasing a Sig P365XL...the 507k is designed for the micro-compact pistols...

ROCK6