Somewhere along the line, it sticks in my mind that the rate of twist was different for slug barrels vs spreader barrels. There's also the fact that shotguns with one spreader barrel (often both barrels are short as well) are referred to as a "becassier" model. A woodcock special.

I have a Verney-Carron catalog from 1999. There are 2 OU's specifically listed as "becassier" models. On both, the lower barrel is described as "raye dispersant" (spreader barrel). It's interesting that their 12ga "gros gibier" (big game) model comes with both barrels cylinder.