If it was made in Belgium for Manufrance, it should carry a belgian proof mark. Also you have to establish that Manufrance was in business before 1898.
The belgian proof if present probably should NOT have a crown over the ELG (introduced on export guns in 1893). The 8 mm Lebel was introduced in 1892, which gives you 5 years of wiggle room if the gun is that caliber.
cheers mooncoon