This rear portion of the sidewall close to the ‘Fwd sidewall liner, and thus form together one whole.

The side has a vent and also a space in which certain things may be storing such as documents or the gear pin.

Explanation + pictures of the construction of the ‘sidewalls’ is to be found at the following link :
http://www.flightdeck737.be/hardware/platform/zijpanelen/constructie/nggallery/page/1