I've managed to clean up some of the rust and looks a lot better but there's still big patches that have gone down to the steel as expected. Is there a way I could tell if it's butted? I've flicked the tubing a few times and the noise does seem to change in the middle. Here's a link to more photos.
https://1drv.ms/f/s!AsQuGx7x47InnANNUttIo4WLU8zf