Thanks Steve. I sent him an email with a link to this thread.
Here's a post by Mike 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. No mention of Stainless.