I believe the barrels on those guns were coated with something like a "black chrome" finish. I had a set of Ithaca/SKB 100 barrels rust blued, it proved to be rather difficult for the gunsmith. He could not get the barrles to rust/black evenly and the mono-block joint remained a silverish color after many rustings. I believe the steel has a high chromium content, thus making rust bluing pretty difficult. I'm not a metalsmith or metallurgist so I'm just speculating here.