I had been sick for a few days and am finally feeling human again. I had just a little bit of time at the gun bench tonight and have started oiling down the Colt DOE upper receiver and looking at it in detail.
In the coming weeks I will do many detailed photos showing you unique features of the 9mm design compared to 5.56 upper receivers. Here is the first photo which gives you a look inside the upper receiver and at some of the chamber area. 5.56mm is on the left, 9mm DOE is on the right.
Quickly notable differences between the two designs is the presence (or not) of a gas tube and the cam pin recess.
I have a busy weekend with the family coming but hope to get something posted for you.