Sounds to me like someone wanted to reduce the choke in the left barrel. While it is a lot more choke, it is along the line of Winchester's WS-1, Remington's
SKEET, and Savage's skeet cylinder for their Ansley H. Fox skeet guns. There exists an Ithaca drawing for such a style choke --
Choke For Skeet Gun.jpg
but I've not measured an NID skeet gun that has it.