I did find out that Brownells does not have an E type stock in the works right now. All they will have at this point is the D type. These will be correct for the following builds:

Model 603 - XM16E1 and Early M16A1 (circa 1967-1970)
Model 604
Model 605
Early Model 651/652 (later production had the E type starting in 1971)