Not entirely true.
For the longest time the only place to get AR parts was from Colt's or surplus military parts, both I would say are "true mil-spec". Only in the last 14 years has this changed.
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I only began the deep dive into the minutia a few years ago and I don't have an engineering background. Just an interest for the details and a few reference books. I just recently got Black Rifle by Blake Stevens!
Weren't the early Colt AR 15's large pin lowers with semi auto carriers?