According to Brophy in his book on Marlin Firearms, Model 90's made prior to 1949 have the rib between the barrels. However, there must have been a transition period because I have a 16 ga. with a "D" serial number ( 1947) that has the rib and I have a 12 ga. with "D" serial number that is open between the barrels. My "G" serial number Model 90 made in 1950 is open between the barrels.