I received today a nice black Corona II in ADG, which appears to be from the late 1960's or early 1970's if I judge from some details like the round rubber legs, 20 holes on the bass side, gold-only colored logos, riveted thumb strap and keys stamped on the bass side strap. However, the keyboard is made of wood with a metal backplate on which the thumbstrap is riveted. The treble side grill is chromed and has a fine mesh cloth. So it doesn't seem to follow the models presented on Peter Unbehauen's web pages on the evolution of the Coronas, it looks like it's a mix of older and newer features.
Sounds really nice and powerful, the bellows seems to have recent tape installed. It's fun to play, now I know why the Coronas are so popular!