Larry,
There are two even scarcer E121-80 back types. See here (Type 3)
http://www.net54baseball.com/showthr...highlight=e121
and here (Type 4)
http://www.net54baseball.com/showthr...highlight=e121
Plus, there is the reversed Type 1 back.
My own view is that these backs will never achieve the kind of premiums you see for difficult T206 backs since (1) interest in this set is much less than T206 and (2) the differences between the back types are relatively subtle.
Scot