Dual res disc master is not correct for 66 with discs or drums... would have been a single pot with a volume extender sleeve for discs, and no extender for drums.
No manual discs system back then (or perhaps ever??), but many cars were manual drums with a single pot. Power brakes were an option most of the time. Dual res master on manual or power drums came in 67.
Likely manual drums originally came on this car, with a later model dual res master bolted up for a good safety upgrade. Could even have later year discs up front retrofitted if the master's bore is suitable.