Could it be 'No. 8'?
https://www.shotgunworld.com/threads/rival-no-8-marked-side-x-side.455471/

With full size in focus close up images of every mark on the barrel flats, just forward of the flats, and action flats we might be able to ID the maker; likely Louis Muller of Liege. He registered twelve tradenames 1889 to 1894 including: Acme, Club, Climax, Star, Rival, Standard, Meteor, and I.X.L.
http://www.littlegun.be/arme%20belge/artisans%20identifies%20m/a%20muller%20gb.htm