@patrick911
You are welcome. Your work helps for mine about the 928 S4 Club-Sport's development, so it is a pleasure to share what I have found.
I'm convinced that the letters after the internal designation are the acronym of the service in which the car is used.
And my guess is that first E stands Entwiklung (development).
It seems that the code was shorter during the '80s.
Examples:
B22 EVA, a 924 Carrera GT.
F8 ERE, a 959 prototype, base on a 930 body.
H21 EPA, a 928 Convertible prototype.