The plot thickens. No idea as to the source of Bro. Landis' information

Twenty-Two Caliber Varmint Rifles by Charles Landis
https://books.google.com/books?id=GUd9CgAAQBAJ&pg=PT394&lpg
The original Winchester Model 21 double barrel shotguns of 12 gauge were made with frames and barrels of 3% nickel steel. Those 3% nickel steel Winchesters were very good, remarkable strong...but wore out machine tools. So along came Winchester Proof Steel, which was just as tough and probably as strong but not as hard...

He is mistaken regarding the hardness of Chrome Moly steel.

AISI 2330 Nickel is 3.25-3.75% with .30% carbon

A post by Mike Hunter in 2015
https://winchestercollector.org/forum/general-discussions-questions/receiver-steel/
Winchesters Nickle Steel had 3 1/2 % nickel and .30%-.40% carbon. I know that Winchester sourced this steel from the Midvale and Crucible Steel Companies.
He states that Proof Steel was 4140.