Is the tenon on the mil spec standard or wide?
I was looking at Millett's site and they mentioned 2 different widths.
It's neither. Springfield's have a proprietary tenon width that is about halfway between Colt's wide and Colt's narrow:
-Colt wide: .125"
-Colt narrow: .058"
A wide tenon will work if you narrow it. You can use a file, but you must be fairly precise, or after staking, it will eventually work itself loose. When doing Springfield's I usually narrow the tenon on a mill. King's makes Springfield, or medium tenon sights.