If I remember correctly, about 20 or 25 years ago there was a short-lived fad of using straight-rifled barrels for trapshooting. There was a theory that even in a smooth bore barrel the wad would rotate some due to the direction in which the bore was polished. This would create flyers in the shot pattern, and decrease pattern density. The straight rifling would ensure the wad did not rotate, and make better patterns. Custom barrels could be ordered, and I think Hastings was one of them. The 2.75" chambers and vent rib suggest to me you might have one of these barrels.
I think it is pretty cool and I would look carefully before selling it off too cheap. If the barrel you have is one of these special trap barrels, it is pretty rare.
Hope this helps :~)