With all the Winchester and Model 12 research, it is odd that there is still confusion (on the internet anyway) regarding the use of nickel steel for only the M12 barrels.

This is the first Model 12 ad I could find in Sporting Life; August 23, 1912 "nickel steel for all its metal parts."



October 19, 1912 ad similar - "all metal parts throughout being made of nickel steel"
https://digital.la84.org/digital/collection/p17103coll17/id/25620/rec/483
The same ad was in the Oct. 19 Forest & Stream, and was the first Model 12 ad I found
https://books.google.com/books?id=nUkcAQAAMAAJ&pg=PA503&vq

November 8, 1913 16g "nickel steel construction throughout" and "twice as much tensile strength as ordinary steel"
That would fit AISI 2330



January 31, 1914 12g



Possibly Researcher has an early Model 12 listing Ordnance Steel barrels, but all the ads in Sporting Life list nickel steel

https://sites.google.com/a/damascusknowledge.com/www/home