The main thing with the old model 12's is the abundance of full chokes. Given the types of steel used for the barrel, typically some kind of Nickel steel in that age, the full choke could be a problem with buckshot or slugs. Your 18" model avoids that. Does it have any choke? Imp. Cyl. perhaps? Regardless, the barrels weren't designed to accomodate steel shot and would likely be scored by that.
Later, barrels were made with Winchester Proof Steel, which I understand to be some kind of chromoly like 4130 or 4340. The one's I've examined seem to have thicker, heavier barrels, but this would do nothing to make them suitable for steel shot, and the choke would still be the limiting factor for buckshot and slugs.
Is it a 12 ga? I think the earliest ones were all 20, but they were probably making 12 ga by 1913 or '14.