This quote came from a post by Dave Noreen (Researcher) on either DoubleGunBBS or the Fox Collectors Forum:
"L.C. Smith changed the 20 ga. from 2 1/2" to 2 3/4" in 1936, my L.C.Smith 16 ga. circa 1939 has 2 9/16" chambers. A 1938 Fulton was still 2 9/16"." I can't confirm the change date to 2 3/4" for 12g but this thread covers Fox, Parker, and Ithaca: http://www.16ga.com/forum/viewtopic.php?t=1983