The purple wires go to a stud on the side of the ignition switch. One of the wires ends up on the red molded connector that goes to the master cylinder; the other wire goes to the gauge.
I can't figure out why they routed it to the ignition switch, unless it was intended to be part of the proof-out circuit. With the 67 ignition switch, there is no proof-out circuit built in. I suspect it was a remnant design feature that Ford abandoned at the last minute in the ignition switch.
I am not sure I would describe the wiring the way you have (underlined). "Master cylinder", I would have used "'Brake Light switch' (on Distribution Block)" and "gauge" I would have written '"Brake" warning lamp (in cluster)"
Otherwise, I have confirmed on a dissected harness that the
black purple wire goes to both, from the pair of wires at the spade connector, connected to the ignition switch.
I might add that the circuit provides a "ground path" for the brake warning lamp in the instrument cluster, to help describe "function".
FWIW, at the 4-wire red molded connector (under dash, at firewall, engine harness to underdash harness connectors), the
black/purple wire changes color in the engine harness to lavender/white.