I was thumbing through Eugene Sloane's "All New Complete Book of Cycling," c. 1980, and in the chapter on chains, there's a photo of a Shimano quick link that looks remarkably similar to the ones we use today--with a wide hole closer to the center of the link, narrowing as it goes toward the end of the link.

I never would have though the quick link concept went back at least 42 years. Sure, BMX bikes and a few others had "master links," but the no-tool-necessary quick-link concept apparently goes back much farther than I would have thought. Except for the one in the photo, I have never seen a similar link in any shop I worked at.

This is on my '84 Ross Mt. Whitney

That's the one!

The little wing you see on the inside of the left link is what keeps it together.
Until the link is twisted 90 degrees, the opposite link can't move in and release the cross pin.
Since they disappeared, I'm assuming that it didn't work well in practice