The detail under the DF antenae whihc you thought might be a gun is the window structure. There was a trangular window made up from the geodetic structure on each side of the fuselage.
Do a google image search for Vickers Wellesley and you will see it much clearer on photos and models.
Perhaps you can read French better than me:
http://www.airwar.ru/image/idop/bww2...ellesley-2.gif
A well finished model of a Wellesley
http://www.google.com/imgres?imgurl=...%3Divns&itbs=1
I explect that the third seat is inside whre the windows are.