The Oval with ELG and a star is a Belgian marking that means the gun was Liege proofed. The crown above it means that the gun was pre-1877. There may be a R with a crown above it also on the barrel. This will indicate a barrel that has a factory original rifling. Do you have a 4 or 5 groove rifling? During this period, many guns were pinfire weapons and there would be a small cutout for a cartridge that had a pin sticking out of the side. I can't tell from the picures if that is the same here. The gun will be chambered in mm and not caliber. Typically there were 2mm to 12mm weapons. There should also be a makers mark on the barrel that is the same as on the receiver. Typically it will have a star above the mark.
Very nice gun, would like to see more of the receiver and the chamber area.