<---- Power Brake Master Cylinder
Production master cylinders for 1969-1970 Mustang/Shelby with power disc brakes are identified by a dimple near the front outlet (1), Bendix logo cast into the bottom (2) and a dome hex head bolt (piston stop) on the bottom (3) The master cylinder, brake booster and cap were Painted semi-gloss black as a whole unit. Plugs were used to keep spray paint from the threaded outlets so you will see a round bare steel circle around each exit.
In the upper picture (original 7K mile Shelby) you can see that it didn't take much brake fluid to remove the light coat of black paint on the master cylinder itself although a white paint dab did survive.
Boss 429 only notes: Boss 429 cars had a unique master cylinder with a smaller 15/16" piston bore, larger outlet ports and a hex headed stop screw. They also had a unique brake pedal and support identified by an spray painted orange dot.
Here are some notes from Joe Way our now retired master cylinder rebuilter:
2227161 is the casting number on the bottom of the 1" bore cylinders, and that is the cylinder that restorers sent me identified as for the standard disc brake cars. It has 3/8"-24 and 1/2"-20 outlet ports.
2227091 is the casting number for the 15/16" Boss 429. It has 1/2"-20 and 9/16"-18 outlet ports.
Note, however, that the same casting was used for some other applications. I have seen the 2227091
casting with 1" bore and with 3/8"-24 and 1/2"-20 ports. I once converted one of these to the Boss 429 configuration by reducing the bore size and machining the outlets.