The 18 is a Belgium mark for bore diameter being 18mm or .71", so pretty well a 12 gauge. Just a hair shy. You could use modern 12 gauge wads in it.
This might help you look closer in specific areas for other marks.
http://www.shotguns.se/html/belgium.html Probably going to be between 1810-1853. Later models usually had London in large scroll letters on top of the rib about mid barrel. That means it is a London pattern. Or patterned to look like the popular English shotguns. The coill spring look is from twisting damascus wire and welding it. It was refered to as London Twist and later London Fine Twist.
Most were farmer grade shotguns sold in Hardware stores in America. For what ever reason the majority of them are right around 1850-1855.
I really can't remember what they called the manufacturing process but 1 shop or smith didn't make the gun in most cases. One fellow would make the lock, the other the hardware, another the stock and so on. Then they would be assembled with a good proofed barrel. The price was much cheaper than the brand name English gun next to it in the hardware store. They were good shotguns and they even made some very good high end shotguns. The brand name generally doesn't matter. If they had one at all it was usually just one similar to a good English firm, to try and capitalize on a known good brand name. In many cases English companies would use Belgium barrels because they could get them much cheaper than making them themselves and were known for their strength. There will be proof marks on it somewhere unless they were rubbed off. That will help you pin down the year. Honestly the brand probably doesn't matter. Let's say 15 villages were putting out shotguns when that one was made. 4 villages may have produced 1 specific brand and once they met their contract they may swap to another brand or no name at all. What matters are the proof marks, that it was actually test fired with an over pressure load without blowing up. And the current condition if you intend to shoot it. One in good shape is safe to shoot with black powder. I have a few of them and shoot them often. I proofed them myself with a string and some distance before ever attempting to hold and fire them.
