The 1925 Winchester catalog No. 83, 1925, still states the 16-gauge Model 12s are chambered for 2 9/16 inch shells and the 20-gauge Model 12s are chambered for 2 1/2 inch shells.
The next small catalog I have I can't find a date on but it is Form No. 980. On the Model 12 page it states "Constructed throughout of Nickel Steel..." Near the bottom they state "The 16 and 20 gauge guns are increasing in popularity at the present time with a large following of sportsmen who find that shooting with the smaller gauges adds much zest and keenness to their sport. The 20 gauge Model 12 is now chambered to handle standard 2 3/4 inch shells." No mention of the 16 gauge chambering.
Riffle states in his book, page 47, that during 1926 Winchester announced that the 16-gauge and 20-gauge chambers would be changed from 2 9/16 and 2 1/2 to 2 3/4. He gives the serial number range for 1926 as 423,057 to 464,564.
Stadt states in his book, page 106, that the 20-gauge was changed to 2 3/4 inch in 1925 and the 16-gauge to 2 3/4 inch in 1927.
The next catalog I have is July 1931, and states all gauges chambered for 2 3/4 inch shells. No mention of Nickel Steel, Stainless Steel or Winchester Proof Steel.
So, guess we can take our pick.