Hell, that's one rough lookin' bit of gear!
Looks to me like the shottie tubes have been cut off a couple of inches forward of the hinge pin and a set of riflebarrels have been sleeved in.
You can see what looks like a join where the shottie tubes have been lopped and also there is a step in the quarter rib at the same location.
Possibly the thing still retains the shottie rib up to the join and then a higher rib section has been added!!