The pics of the OP's BP looks like nickel that is just tarnished and scuffed. I've seen many nickel guns with heavy tarnishing and heavy scuffing that would resemble stainless to some people. Notice how the cylinder is hazy or "milky" looking which is typical nickel tarnish. As mentioned above, the BP MKIII was offered in blue and nickel. The blued versions were more of a matte blue and weren't polished as well as the Trooper MKIII sisters similar to the S&W model 28 to it's model 27 sister. I've seen some nickel BP's where the nickel was more of a matte nickel also and not as mirror bright as the Troopers. The BP's weren't polished as much as the MKIII's.
Colt was known for the best finishers in the industry but finishes varied from gun to gun, year to year and polisher to polisher. Just like some Pythons are more of a deeper, brighter blue than others even though they were all Royal Blue the finish varied. Even though the BP's weren't polished as much as the Trooper MKIII's, the cylinders and barrels were usually brighter than the frame because of the harder treated carbon steel but, they still weren't polished as bright as most of the Troopers. I can see where the owner could mistake a tarnished, scuffed nickel BP (which wasn't polished as much) for stainless. I'd bet that nickel would polish up nice.