Push the pin out at the rear of the flip up sight. Remove the spring by lifting up at the front and sliding it forward. Slide the flange nut into the under cut at the front of the sight where the spring was. 

Place the mount over the flange nut with the (2) 4-40 tapped holes toward the but end. Line the 4-40 tapped holes in the mount up with the holes in the side of the receiver. It is very important that these holes line up to keep from damaging the threads when the screws are installed. 

Put in the (2) 4-40 socket head screws. DO NOT TIGHTEN!! Place a pin drill etc. through the countersunk hole in the top of the mount and into the hole in the special nut. Move the nut around with the pin until the round part of the nut slips into the counter bore in the bottom of the mount. 

Put the 10 – 32 x ¾ flat head screw through the mount and into the nut. Tighten this screw and then the (2) 4-40 screws. You don’t need to get the (2) 4-40 screws very tight! Put the 10-32 x ½ set screws down through the tapped hole in the top of the mount and tighten against receiver. DO NOT try to remove the (2) 4-40 screws without first loosening the 10-32 front set screw