It's a Beretta tactical pistol mount from my PX4. It had accommodations for 2 small pieces of Weaver rail on the sides and the bolt holes just happen to correspond with the butt stock mount and sit it just slightly off the dust cover and seeing as I've never used the rail as it makes the pistol almost impossible to holster, I cut the bottom off and bolted it to the butt stock.What exactly is that you've got your red dot mounted on?