Thank you Marc for the confirmation, and information about the one on Holts auction. No, I will not bid.
Looking in both catalogs yesterday evening, it was the boxlock who was stated to be created 1905, this both in the Branquart and Defourny catalogs. Guess the sidelock come one or few year later.
Here is a link to a similiar boxlock gun, it's nr 50, http://www.littlegun.be/arme%20belge/artisans%20identifies%20d/a%20defourny%20gb.htm