Re: 1892 Winchester 32-20
Without seeing the gun (and possibly even then), it would not be possible to be sure whether the barrel is factory, but it well could be.
In that era, Winchester would install any length barrel the customer wanted. Extra length barrels cost $1 per inch, IIRC, and short barrels, down to 12 inches, were available on special order. Your 18" barrel is unquestionably legal and, if factory, will bring an extra premium, but not a lot.