accoarding to pelican parts the 911 (up to 89) MC is only 19mm but maybe the volume is more??
and if using the 944 MC which one? there are like 4. EXPENSIVE!!!
Master Cylinder For Use With Girling Brake Booster, 944 (1983-86), 924S (1987-88), Each [Photo]
E-355-011-00 $221.55
Master Cylinder For Use With ATE Brake Booster, 944 (1983-86), 924S (1987-88), 944 Turbo (1986), Each
E-355-011-01 $140.20
Master Cylinder For Use With Girling Brake Booster, 944/944S (1987-88), 944S2 (1989-91), Each
E-355-011-10 $258.30
Master Cylinder For Use With ATE Brake Booster, 944/944 Turbo (1987-88), 944S/S2 (1987-91), 968 (1992-95), Each
Also maybe this helps? no pics though. I kow that jettas and stuff have a proportioning valve that basically screws into the MC wonder if this is the same? too bad its NLA.
Brake Pressure Regulator, 944 (1985-91), 944 Turbo (1986-89), Each