C = 60
D = 70
E = 80
7 = 7 easy enough
E7TE = 1987 engineering number. Means the part was designed/engineered in 1987. The T most often signifies it's for a truck, but not always. The next cluster of numbers indicate the type pf part it is (ie... head, intake, exhaust, etc...) then if there are more numbers they are revisions.
So an E7TE head with PA is just a revised E7TE head. Could be something as simple as adding a bolt hole or as major as a different valve size or anything sort of a total redesign.