There are 8 M21 double rifles in the upcoming Julia auction. They were built by CSMC and have consecutive serial numbers and are in Grand American style. Quite impressive.
I didn't realize CSMC built big bore double rifles but they obviously did---the 470 nitro express is the largest caliber.
I recall Winchester built a few double rifles and there was one in the Cody museum a number of years ago---may still be there.
Have a look see and try not to drool.