The NID period Ithaca catalogues up thru 1933 (Catalogue No. 50)still state 16-gauges are chambered for 2 9/16 inch shells. Beginning in 1934 (Catalogue No. 51) they state that 16-gauges are chambered for 2 3/4 inch shells.
The 2 3/4 inch 16-gauge shell got it impetus from Remington introducing their 16-gauge autoloaders, Model 11 and The Sportsman, in 16-gauge chambered for the 2 3/4 inch shell circa 1931. Remington then also introduced their 2 3/4 inch Auto-Express 16-gauge shells with 1/4 dram equiv. more powder but the same 1 1/8 ounces of shot as the 2 9/16 inch Nitro Express or Super-X.
The Lefever Nitro-Special was introduced by Ithaca circa 1921. Serial numbers began at 100000 and by the end of 1933 were up to 298749. The highest serial number in 1947 was 361199, and in 1935 a block of 25000 serial numbers (300000 to 325000) was set aside for the Lefever A Grade. So it is very likely more Nitro Special 16-gauges left Fall Creek with 2 9/16 inch chambers then with 2 3/4 inch chambers. Give us the serial number and we can give you the year of this Nitro-Special from the table in Walter Snyder's book.