No, not so rare. What happened is that Ford were not expecting to have to use the 3.3 in 1979 to 1982 Mustangs.I'm guessing, but the E1 possibly had a casting number change for federal indentification. The later Fairmont/Cougar/Capri/Aero LTD had a low mount starters to cope with the detachable bell housing C5 transmissions. Last Fairmont was made in 1984.
Ford is real funny. Cost implications make day to day decisons.
I'll bet there were two E1 castings, such as E1-6015-AA for the high mount, and seperate revision code for the low mount. Jack said that any two cars could have whatever engine blocks were sent over lake Michigan from the engines stockpiled at the Windsor, Ontario plant where they were made.