The pics you have posted are of some help...but not enough to know what it is.
More pics, better pics and of different angles would sure help. There should be something stamped somewhere on the gun that are sometimes hard to locate and see. I use a magnifying glass and different lighting to find them.
I did find a shotgun that resembles yours but it's not an exact match but it's very hard to tell for sure. It does have many of the same features.
The gun is Belgian, one of many millions of Belgian shotguns sold throughout the world. That one appears to have solid steel barrels (not Damascus) and probably dates to around 1910-1920, though some others may contribute to the age issue.
Similar guns were manufactured in Belgium for Crescent for about the first 1/3 of the Twentieth century. They were utilitarian guns of reasonably good quality.
It is 12 gauge, but is chambered for the old 2 1/2 inch shells, not the modern 2 3/4" or 3" shells, and the longer shells should not be fired in it.
"Seresien steel" means that the barrels were made of steel from the steel mills at Seraing, Belgium. The actual mill is not indicated, so the marking is equivalent to saying "Pittsburgh Steel".
Value? If you have it checked out by a gunsmith and it is safe to fire, maybe $200, low partly because 2 1/2" shells are hard to find today. Whether the chambers could be lengthened would be a question for your gunsmith.