Interesting. The Stoners obviously had 30 rd mags as is evidenced by the Stoner mag pouches in the last photo. I wonder how an effective 30 rd mag could have been developed for the Stoner in 1966 but it took until about three years later for Colt to develop a reliably functioning 30 rd mag for the AR?
I have two of the Stoner mag pouches available if anyone is looking for them.
The Stoner 63 started life in 1963 with a 30 round magazine. In fact, they never had anything but 30 round magazines.
The AR-15 was a 1957 design, when everybody figured 20 rounds were plenty. (Hey, it's 2-1/2 times more than an M1!).
In 1964-ish, Colt developed the CAR Automatic Rifle for the SAWS program, a BAR or M15 type version of the AR-15, these had the early full curve 30 round magazine, which performed very well during the SAWS testing. Unfortunately, the M16 had sloppy tolerancing on the length of the magazine well, and the simple, reliable, full curve magazine would not fit all M16s, so they had to go back and design the compound straight-curve magazine we have today. Then they had to run it through exhaustive tests, and then manufacture them in quantity for fielding. Testing wasn't complete until autumn 1967, so production probably only started going in earnest in early 1968.