SL was a trade mark for "Syndicat Liégeois pour la fabrication des armes de guerre" ( Liege Syndicate for the production of arms of war) . It was established in 1884. As the name implies it was not a true company was composed of several makers:
Jules ANCION & Co.
DRESSE, LALOUX & Co.
Auguste FRANCOTTE.
PIRLOT & FRESART

http://www.littlegun.be/arme%20belge/artisans%20identifies%20s/a%20syndicat%20liegeois%20gb.htm

Pete