1. Crown-ELG means made in Belgium after 1893, black powder proof test.
2. 16 C in a diamond means 16 gauge made between 1898 and 1924, black powder proof.
3. A "fancy L" means the barrel was tested rough before the gun was finished, 1924 and after.
Combination of 2 and 3 might mean the gun was made in 1924 when the marking systems overlapped. Unless the "fancy L" is really a fancy EL which was used earlier.
4. 17 is the bore diameter of a 16 gauge in millimeters.
Now it gets interesting.
If the other barrel is WAY smaller... like 10.7mm and if it is really rifled; that makes it a Cape Gun, combination rifle and shotgun. Is the small barrel rifled? Does the gun have rifle type sights?
Now, it is only a black powder gun, the 16 gauge chamber is shorter than modern 2 3/4" and heaven only knows what rifle round might shoot through a barrel that the proof house gauged at 10.7mm. There are several different European 10 to 11 mm cartridges to consider. A gunsmith could do a chamber cast and figure out which it was. Ammunition would not be available, it would be an expert expensive handloading project.
GOOD pictures would help a lot. Phone pictures won't do, it needs a high megapixel camera with a closeup setting.