Post pictures of the proof marks and we should be able to tell exactly when it was made. On a side note. A 28ga is no longer allowed for hunting in Belgium which is ashame.
http://damascus-barrels.com/Belgian_All_Proofmarks.html

Adolphe Jansen was a royal gun maker. That is he held the title of Gun Maker to His Royal Highness. That does not mean that every gun produced by him was owned by the king

You can read a bit about him here:
http://www.littlegun.be/arme%20belge/art...dolphe%20gb.htm

Jansen or Janssen was part of what became a large, influencial family of gun makers in Belgium:
http://www.littlegun.be/arme%20belge/artisans%20identifies%20i%20j%20k/a%20janssen%20gb.htm

Over the years, they developed business partnerships with many other Belgian makers.

Pete

http://www.damascus-barrels.com/Movie.html