Louis Flobert of Paris was the inventor in 1857 of a type of rimfire cartridge using only the primer to propel the bullet, like the modern BB Cap. He also invented a system of rifles and pistols in which the hammer served as both the firing mechanism and the breechblock. The combination of the hammer mass and the momentum from its forward movement served to prevent the cartridge case from blowing backward. If a cartridge is used that is more powerful than Flobert's, the cartridge case will blow the hammer back and fly out, possibly injuring the shooter.
Warnant, a Belgium, invented a breech block that could be used on Flobert type guns but which would allow use of a (relatively) more powerful cartridge. That is the system used on the gun in question.
These were rimfire, NOT MADE TO FIRE ANY ROUND WITH POWDER. They are typically metric, around 9mm. The ELG in the oval is a Belgian proof from Liege.
BTW, welcome to the forum. Good folks around here. When you get a chance, stop over by intro section and say howdy.