One difference is that the CCO has a mild chain link texture on the front strap and MSH, while the ECO has 25 LPI checkering.
Another difference is that the CCO has Novak-style three dot night sights, while the ECO has Heinie Ledge Straight-8 two dot sights.
One more difference is that the CCO uses a standard GI guide rod and barrel bushing setup, while the ECO has a bull barrel and full lenght guide rod.
I'm sure there are more, but these seemed to be the biggest differences I can think of at the moment. I really don't consider any of these differences to be definite advantages in favor of either pistol....merely differences that may be either an advantage or disadvantage on personal preferences.
I just thought of one more, that's not overly important, at least to me. The top of the ECO's slide incorporates a rib.
