For clarification, are you talking about the beefed up part where the RE threads into the lower? I would be shocked if any Colt didn’t have that.
In fact Ar15 has different receiver reinforcement.
You lost me there. Are you saying Colt is using different forging platters for different models/producion runs? Or other AR15s are different forgings?
Or it could be from two different forges on the lowers and debating on the machine it was cut on at Colt it could be +/- of a certain spec.
Also are you sure the Colt lower/upper you have was from Colt and maybe on one of the few Colt holding companies that ran production lowers with Colt’s Rollmark?
This is a curious characteristic to focus on. I mean… I couldn’t tell you what that portion of any of my many lowers looks like in much detail. And I’ve never seen a lower fail there unless it was runover or something.