There is more to the story than just a matte finish on the topstrap. That gun has been in contact with something that caused serious rusting of the entire gun. (Yes, "stainless" steel can and does rust if it comes into contact with some chemicals or if left in a wet environment with no protection.) That corrosion might be too deep to be easily polished out, but I agree on leaving the top strap and rib alone while the rest of the gun is polished. Then the parts that were matte finished can be done as others suggest, or simply left alone.
Jim
Jim