Not having a go or anything, because admittedly, I'm not a brake specialist, and never taken either of the mater cylinders apart. But it is possible the the part numbers correspond to the casting of the part, and don't relate to the internal set-up in anyway. I believe the disk/drum and disk/disk run the same master cylinder bore. Also, having driven the car with the base set-up and the S2 disk set-up, I'm very thankful to have the better brakes - I've used them on numerous occasions, and although I still struggle around in a straight with the carby anchor up front, its still the reassuring fact that if I go into a corner, on a twisty road well under the posted speed limit, that the car will pull up straight and true every time. But each to their own -it's up to the individual if they want to do it or not.