Some archival photos...

[Robert Goodman]

Originally Posted by Forksref

the wing would signal with a fist (much like punching back a backward pass) toward the EZ that the runner had crossed the GL,

And they would "sell" that call with a repeated and/or exaggerated "punch" if it wasn't obvious. I've seen film of such a case (ball crossing plane, runner landing in the field of play) where you could see both wing officials facing infield and swinging their fist from the field-of-play side arm over their head toward the end zone and back, at least twice, like they were mimicking the action of the ball.

Last time I saw this signal was on TV in the Hula Bowl in the 1970s, and the official used an open hand; looked like a rugby ref giving a try.

There's a remnant of this practice in an umpire's fist-with-thumb-up signal.