Maybe he is referring to the true "Colt" M4 lower receiver. Colt puts matching "cuts" into the lower receiver to match the feed ramps on the barrel extension. This, in theory, aids in reliability, but I don't think any other manufacturer does this to their lower receivers. Other than that there is no difference other than the rail markings on the flat-top A3's. There really is no difference. An M4 lower can be A2, A3, have extended feedramps cut into it or numbered rail marking on top.