I found this 1984 United Airlines commercial featuring their first-class service on YouTube.
At the :41 second mark there is a shot that shows the arm rest; it looks too wide to fit between the seat up-backs.
The plane featured in the commercial is a 747; I don’t believe the plane this alleged incident took place is identified.
https://www.youtube.com/watch?v=ylkH1bbETE8
It’s a pocket into which a table top fits.