Kurt, I have a 1939 16 ga. Field Grade L.C. Smith that still has 2 9/16" chambers. I believe that Hunter Arms Co. dropped the Fulton line in 1945 along about the time it was being sold to the L.C. Smith Gun Co. a subsidiary of Marlin.
Some say it was the late 30's or early 40's that they changed the chambers over to 2 3/4". Go to this website and see when your gun was made. http://www.lcsmith.org/organization/membership.html
Go down the first column to "Shotguns",opens, go to "Year of Manufacture".

David