OK, here is another take on these, originally called Ashley Express Sights. I first heard about them MANY MANY years ago. This is my favorite type of sight on any kind of firearm. AND if your eyes aren't what they used to be, you will be surprised how much your shooting can improve over the conventional square notch rear sights.

http://www.officer.com/article/10757262/have-a-look-sight-systems-24-7-big-dot