Oscilloscopes of this era (excluding "synchroscopes") had a synchronized time base, rather than the triggered time base that we all expect now (analog or digital 'scope).
The time base is an astable oscillator, whose free-running frequency is determined by the "coarse" and "fine" frequency controls on the front panel. The "sync" signal (through the "sync amplitude" control) restarts the oscillator so that its actual frequency gives you a stable display.
Interesting that this unit has only one-range "gain" controls for both axes (no stepped attenuator).
Is there an internal connection for the missing binding post for "Z-axis" (brightness modulation)?