What were the 20 airspace classes in the US before 1993?
I don't claim to have a definitive answer, but started listing the airspace I recall memorizing or seeing on charts.  So here's my current list:


Continental control
positive control area
Air defense identification zone
Oceanic control area (may not be USA)
terminal control area
airport surveillance area 
tower control area 
transition area
control zone
uncontrolled airspace
Military operational area
prohibited
restricted
alert area
warning area
caution area (1943 aeronautical chart)
danger area (1943 aeronautical chart)
airways
airport traffic area
national park 
wildlife refuge
fire control area (became restricted as I recall)
special use area
laser demonstrations
blast area (later warning area, now TFR)
controlled airspace