Most Adjustable Colt's used Ellison sights in those days. Millet made drop in leafs for them and many others. Since they are both out of business, you have to search ebay and gun shows for parts.
Commanders didn't come with Elliason or Colt Accro rear sights unless it was a special run such as a Gold Cup Commander...not many made. More than likely it came with the standard fixed sights and someone added the Millet. Elliason sights comes on Gold Cups and some target model Pythons. Colts that came equipped with factory adjustable rear sights had the revolver frame or slide milled to accept it. The Millett in the photo is one designed to fit the standard fixed sight dovetail. It must have an aftermarket front sight as well to match the rear sight's height.