That is the correct mag. They issued all they had left before starting out with a new post-war mag. Your mag was refinished for military issue but was left over after the war. Originally they had a satin blue finish. They must have had thousands of the base plates on hand too, because they also used them to make welded mags during the war. So, you can find several types with the same floor plate; satin blue, pinned, original pre-war GM, matte pinned, WWII production, matte welded, WWII production.
BTW, that '47 is beautiful!