Found a 1959 ‘Guns And Ammo’ article showing off the futuristic Armalite AR-15. It’s still listed as chambered in .222 Remington, but is vaguely indicted to be using “hotter” rounds. It wouldn’t be until 1963 that 5.56mm, which is based on the .222 is officially recognized.

This early AR-15 has a number of unusual features such as the trianglur tab shape for the charging handle, the slot on top which is probably a legacy from the top mounted charging handle (maybe leaving the design open to reintroducing it if a customer wants that), the sling connecting to the bottom of the pistol grip rather than the stock, the early and unique handguard and flashhider. There’s plenty of other little details that change over time from what we currently think of as “standard” in this early version.

The rest of the article talks more about the AR-15, as well as early AR-10s.