"JP" on the forend catch is the same as the mark used by Jacques Pirotte et Fils. (et Fils means, by the way, "and sons").
As to JBL, this could perhaps be:
Lehanne, Jean Baptiste, 18 Rue du Coin, Herve, (Liege).
or
Lhonneux, JB, (or JD), Liege. Perhaps too early for your side by side.
You'll find the following is very useful for Belgian gun trademarks. Indexed by surnames, so look up "L" to follow up "JBL". However, it does not record these particular 3 letters, just lots more like them.
http://mallorquina.pagesperso-orange.fr/source/pageP.htm Good luck.
