The *S is the Controller of Proof mark. The Controller was in charge, he never touched your gun.

Here are all the Belgian proof marks
http://damascus-barrels.com/Belgian_All_Proofmarks.html

Belgian makers & trade marks
http://damascus-barrels.com/Belgian_Trade_Marks.html
http://www.littlegun.be/arme%20belge/a%20a%20artisans%20identifies%20gb.htm

Pete