I’m assuming the rib is the factory solid rib, many were not. The factory matted rib and 28” barrels were options, the rib was an additional cost option. I wasn’t able to find a numerical answer to your question but I can assure you it would have made a dandy trap gun for a shooter who found the standard 30” barrel a bit muzzle-heavy, as I do. Not common but not rare, I’ll try to find a better answer if someone doesn’t have it at their fingertips. 

Mike