Keyhole in side of MCI 7?

[Tenor]

On both sides of the bus, slightly foreward of the center line between the rear axles, about 10 inches below the side windows are what look like keyholes in the skin.  There is a riveted box behind the opening to keep water out of the walls.  What were these for?  It looks like something would have been hung from them.  Just curious!  Thanks!

Glenn