Presumably chambered for 12g.
13b and 14 muzzle are pretty normal dimensions for a nominal 12g shotgun. With bore, the higher the number, the smaller the diameter, for it is determined by the number of lead spheres of that diameter to the pound, so the muzzle has a smaller diameter than the bore; the bore is smaller than the chamber. 16, 20, 28 etc are getting smaller; 10, 8 and 4 are getting bigger... a hypothetical 1b would be the diameter of a single 1lb lead ball.