Just as an FYI-the 1958 through 1962 Stoegers Shooters Bible shows Sauer drillings, such as yours, as Sauer 3000 Lux-Dur. So I would say that it is a Sauer 3000. The only difference that I have seen between a Sauer 3000 and a Colt is that it is marked, I believe, as made for Colt by JP Sauer. Also, I have only seen Colts chambered in 12x12 over .243 or 30-06. There may have been others but I doubt it.
I suspect that the recoil pad has been added to yours. Nice firearm. Enjoy!