Well, I'm still not sure how you figure this is possible, since the law clearly lists the AR-15 as restricted, but let's put that aside for a moment and focus on the practical aspects.
I am pretty sure that the Colt AR-15 SP1, which was the first AR-15 variant to be manufactured as a semi-auto, originated in 1964 after the US DoD issued purchase orders for the first M16 and XM16E1 rifles. Colts primary motivation was obvious: they had a new rifle that was making waves and entering military service, many average US shooters would want to experience the rifle, so they needed a semi-auto.
There was a second motivating factor. As you suggested, the military did require a few modifications, the most important of which was a new, fully captive front pivot pin. This necessitated a new receiver design and new forging dies, and the DoD would not accept any new rifles built on old receivers. This was a problem for Colt, because they had about 100,000 forgings of the original receiver pattern in stock, and they could not be modified. So the SP1 was introduced, in part, as a way to use up those old receivers. It took Colt until about 1982 to use them all, and then they switched to the current receiver style (fourth version) and changed the model number. So all Colt SP1 rifles, regardless of age, are built on the same receiver (first style) as those very first, 1964 semi-auto rifles.