Home Lighting Relay Characteristics
1/2 hp is a motor rating.  Motors have a whole bunch of inductive kick, so controlling relays must be de-rated. 
20A is the rating at 277V.  Interesting thing, 277V was commonly used in industrial lighting, where they were interested in a high ratio of lumens to copper (40kw on twelve 12AWG wires); so 277V almost implied HID lighting with magnetic ballasts (sodium, metal halide, etc.)  This is called a "ballast load" because they too have an inductive kick, though not like a motor.  This seems to imply a 20A rating for ballast load.  
20A is the rating at 120V.  120V lighting tended to be incandescents, i.e. a simple resistive load with no kick. Incandescents do have an inrush current which is considerable, so contacts need to be a little tougher than a pure resistive load. By the way, electronic ballasts and LEDs also have inrush current from the capacitors/inductors charging up. 


The 30V DC is approximately what you get when you take a common 24VAC thermostat or doorbell transformer and rectify it to DC.  Very appropriate for a 1960s system.