First I know this is an old post with no reply..
I'm a Newbie here BTW..
I am having the same troubles as you as My N66MB 1959 Model has/had the rear sight windage screw head broken off..
I tried removing the screw without removing the sight assembly from the reciever cover but because there's a small "Hidden" tab on the bottom side of the sight this is impossible..
I went ahead & took a small dremel ball & removed the staked part of the rivet from the underside of the reciever cover & with the rivet head inside a small 1/4" socket & with a punch drove the rivet out..
Went ahead & cleaned the rust etc from the sight assembly & restaked the bottom of the elevation screws tennon washer..
Cold blued the parts to make them look as good as possible for being 50 years old this next month..
I don't have a "New Windage Screw" or "Sight Rivet" YET!!
Wondered were you found yours at ballistx??
I've looked at Numbrich/E-Gun Parts, Midway & Brownell's without any luck.. I will try calling Remington thought..
I took a few pics of the parts to try an show why you can't pry up the rear of the rear sight, there's a "Hidden Tang" there..
If all else fails I'll make a windage screw & rivet..
Lucky for me I have a Bushnell 3x7x18 rimfire scope mounted on my Nylon 66 for now & have a Ruger 10/22 for iron sight plinking..
Gary/HkRemington Nylon 66 Rear Sight Assembly RemovedRear Sight Assembly "Tang" Shown Upside DownRivet Hole in Reciever Cover Shown From The TopThreads on Reciever Cover