The ‘sidewalls’ in a B737 are also equipped with built-in ashtrays (ash-trays). Things that are no longer used, but they are still there.

The ashtrays that I use in my ‘sidewalls to see on the front almost like the typical ‘B737 ash-trays’ but are actually no ashtrays.
The are small boxes in which normal business cards are stored.

More info : http://www.flightdeck737.be/hardware/platform/zijpanelen/inopbouwonderdelen/