Can't answer when exactly, but I'm imagining somewhere around 1983 when EPCOT opened, because I don't remember that ticket structure at EPCOT and the start of the two day pass (also long forgotten)
Yes, there were A, B, C, D, E tickets in a book as well as the entrance ticket (originally $3.95) A tickets were things like Main Street Transportation and were about $0.10 A D ticket got you on Jungle Cruise. E tickets were for things like Haunted Masion, Space Mountain etc.
E ticket has now become a generic phrase to represent a ride that will blow you away, like Mission Space, Soarin', and Expedition Everest.
There is a series of new lanyard pins that show the old tickets, each with a character on it and you can see the rides were for each ticket.