E = export model (XLR connector). Alternatives in the past have been C = continental model (small Tuchel connector), L = Lemo (for the Austrian state broadcast network ORF) and F = France (Sogie connector, yuck).
Since the XLR connector achieved nearly total dominance in the professional market decades ago, and non-XLR versions ceased manufacture except as occasional, special orders, AKG stopped adding the "E" to most of their model numbers. It had become a "distinction without a difference."
More recently, Neumann has cautiously begun doing the same thing with the suffix i = international, Beyer has dropped their (C) = Cannon suffix, and Schoeps could probably do the same with the U = universal at the end of most of their product names--except that Schoeps does use Lemo and Binder connectors for certain products (such as the CCM L series and the recently discontinued CMB I amplifiers), so in their case it actually does contain some distinguishing information.
--best regards