Seam to the left of tray-table hinge for changing seat covers.
The interior of an airplane is built to be re-skinnable. It has to be: carriers change color schemes and planes change carriers. You don’t want your shiny ‘new’ 737 to come with someone else’s seating arrangement or cabin décor, do you?
How much of our interpretation of the built environment is predicated on how things look, not what things are? A fair amount, I’d say.
A rubber strip covers the tracks in the floor into which seats are fastened.
See also: Regional ‘feeder’ airlines (Delta Connection, Continental Express) that are neither owned nor operated by the airlines whose liveries they wear.