It is a twisted story.
AR-10 -> AR-15 ->XM-16
The Air Force fielded the AR-15 (eventually GAU's) while everyone else went XM-16. The Air Force (and Stoner) didn't think Forward Assist was necessary, hence the split. That is the "major" difference between the original (slick side) AR-15 and the XM-16 (there were a few other smaller updates like gates and stuff). If you look at some of the weapons our "advisors" used early on in Vietnam, along with the original Colt Sporter, you will see that they are slick-sides.
https://thecoltar15resource.com/2020/12/20/december-1964-shooting-times-article-on-the-colt-sporter/
There are even original military lowers out there stamped both AR-15 AND XM-16, but that was all during the early transitions.
However, the early AR-15 mentioned above is not the same as the "civilian" AR-15's, with the big difference being the lowers are cut differently to accommodate full auto capability. The semi-only cuts (high shelf vs low shelf vs full cut) were never used by the military as they cannot accommodate the parts needed for select fire function.
Edit, I should add. If one looks, the different cuts are available. But the point was that the "civilian' AR-15's were mostly semi-auto only. However, there were full-auto AR-15's available to the public. While "legal" at the time, I'm not sure if that was Colt's intention or not, being that they were also selling their Semi-auto only Sporter.
It is a fascinating history.
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LOL, look at those EARLY charging handles on the original AR's.