Based on the serial number your gun was made in the late 30's, probably about 1938. Ribs were optional on most prewar superposeds, even though few were sold without a rib. The solid rib was only about a $2 charge, so most guns were ordered with solid ribs. If your gun has the same style fore end as the post war guns (not the metal "horseshoe" retainer) then it is virtually identical with the very early post war guns.
I think most would say the high point of superposed quality was in the late 1950's, not the pre war guns.
The 3" chambers on your gun are unusual if they were done at the factory. Very few 3" guns were sold, and they were a special order item, or could be added if a 2 3/4 gun was returned to the factory. I have looked at quite a few prewar guns, own several, and have not seen one that appeared to be chambered to 3" at the factory.
