I know it's the right model because it is a factory hydroboost spec MC, which I obviously have exceptional familiarity with. I had a '80 diesel Cadillac Eldorado 4 wheel disc factory equipped with hydroboost that ran this exact MC. I yanked the (junk) Type II internal accumulator model hydroboost unit out and swapped in the vastly superior external accumulator Type 1 model, recalling this MC in my hands very clearly while doing this. Looking at your picture, I do not believe you will have to swap out the reservoir - should be spot on - it is a 1" bore. Of course I also yanked the diesel Olds 350 out and swapped in an Olds 455 in its place. What a trip that was watching tire smoke billowing out of the front wheel wells - it would pour smoke right into the cabin real bad if you had the windows down LOL.
As far as RockAuto? I wasn't endorsing them, just mentioning that they have what I feel to be a nice quick reference visual when needed. I have found their images to be accurate in most all cases. I also use NAPA, as we have a MASSIVE NAPA distribution warehouse literally right behind us with a customer showroom and counter. What I have however found with NAPA online is that their images are not as accurate many times. I checked the application just now on NAPA's website and this application IS coming up, but with
incorrect images. I just ran a Bing search for 1980 Cadillac Eldorado Master Cylinder and it produced a whole bunch of listings with the correct images.
See if your NAPA contacts can cross reference any of these:
ACDelco 18M95
Raybestos MC39079
Duralast NM1740
Centric 130.62036 (remove the dot in some lookups)
Dorman M101260
Or? Just order in from Summit Racing:
https://www.summitracing.com/parts/ado-18m95
https://www.summitracing.com/parts/agb-mc39079