What were the 20 airspace classes in the US before 1993? ICAO adopted the current airspace classification in 1990. The US followed suit in 1993.

But, I found this:


  In case you didn't know, the U.S. had 20 different types of airspace designations prior to 1993. (VATSIM)



Were there really 20 classes?
What were they and their functions?
And if possible, how did they compare with the rest of the world pre-1990 in a general sense?


What I could find was the Oct 1993 issue of Flying Magazine. From which, I couldn't find the 20 classifications, just 7 old names and some changes.

The FAA order 7400.2D from 1993 includes the change, but I can't find the similar order that precedes it. The preceding document is here, but unhelpful.

Also via here we find: Transition Areas, Continental Control Areas, and Control Areas. All three are now under Class E. That brings the total to 10 out of 20.






Thanks to @Pondlife for the FAA document and for checking the AOPA magazine article.
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