think, it was made before 1914, it were more different actions around then. after the war the toplever anson gun became the norm.
look on little gun https://www.littlegun.be/arme%20belge/index%20armes%20belges%20gb.htm
the belgian gunquarter was nearby,maybe you find a similar gun.
the spur opening was not completly unusual also on german made break actions before the great war.