Since the name on the barrels is in English, and is a "copy" of a famous English gunmaker (Manton), I suspect you have a gun made in Belgium for export to the US. They were sold in hardware stores by the thousands, fairly inexpensively, to people who couldn't afford Ithaca or Fox guns. With the lever so far to the right, I suspect it's pretty loose. I wouldn't shoot it without an OK from a gunsmith who works on doubles.
You might want to take a good picture of the water table and barrel flats and post it on the double gun BBS. There are some real experts on old doubles there.
This. Not to be a downer, but that’s what it is. Inherited the same one in 10ga (in a pieces, put back together). It’s still a neat little piece of history. These were made with all manner of stampings to “sound like” something “fancy”. No offense intended, do realize how this sounds, but if lay a “real” english, belgian or german, etc next to it, you’ll suddenly see what’s what. Lastly, like Hogeye said, these were of marginal steels, sorry. Personally wouldn’t shoot it, but if you desire, certainly needs a once over by a knowledgeable shotgun smith, not just corner joe blow. Mines hanging in the little cabin as a family memento of the hard scrabble homestead farm years.