1960 Winchester Western catalog
https://cartridgecollectors.org/content/...l%20Catalog.pdf
"Winchester Proof Steel" is only specified for the M42
No mention of barrel steel for the M21 and M50
M12 with Chrome Molybdenum receiver; no mention of WPS barrel

1965
https://cartridgecollectors.org/content/...l%20Catalog.pdf
No mention of WPS for any of the shotguns; M21, M12, M1200 nor M1400

1970
https://cartridgecollectors.org/content/...l%20Catalog.pdf

1972 catalog listed Chrome Molybdenum for the Y Model 12
https://cartridgecollectors.org/content/...l%20Catalog.pdf

1972 magazine ad listing Winchester Proof Steel barrels and Chrome Molybdenum receivers for the 1200 and 1400
https://books.google.com/books?id=gl31ZxmtlioC&pg=PA13&lpg