I gathered this information from a recent thread on the HillmanCars group
[Someone was looking for] … Clutch master and slave cylinders for my 1966 Sunbeam Minx. This car is the same as a 1964 Hillman Minx V. My manual shows the parts numbers as 1223548 for the Master Cylinder and 1223557 for the Slave Cylinder. Any suggestions for parts outlet or a reasonable substitute?
Suggestions are from old notes Richard “Budd” Bennion collected
Take a look at a clutch master cylinder for a Triumph 2000, MGB 1965-1970 (but must swap the operating rod from the Hillman as the MGB rod is too short) or Saab 99-900.
Also maybe check Range Rover 1988-90 as it can work on a Series I Hillman but it is the self adjusting type instead of the manual adjust.
Jan Eyerman replied:
I have a clutch master cylinder for a 1965 MGB in my 1962 Minx Series IIIC and it has worked fine for several years. I did indeed have to swap the operating rod from my old master cylinder as the MGB one is indeed too short.
One other thing I did was to use a banjo fitting on the end of the master cylinder to make installing the fluid line easier.