71 demon master cylinder

The factory ones did have a larger reservoir for the discs. Various aftermarket one's did have castings that look like yours & will work with discs. I would definitely check the bore size & verify it's right, believe it should be 1-1/32" for yours. Also check the connection dimensions, you don't want to have to bend new lines. I had one that was intended for discs where they put in the drum check valve- when you put in fluid make sure it flows out of the disc part but not the drum. Definitely bench bleed it.

Unfortunately all the replacements I know of are from China with spotty quality control. Even the rebuild kits are China sourced. The rebuilt ones seem worse, they seem to throw anything in the box whether right or not. If you ever find a vintage USA-built one, buy it. There are some guys that make a living out of selling old rebuilt MC's that actually work but pretty pricey. The best of all worlds is to have an original stainless sleeved & rebuild it with quality parts.