So in summary:
Winchester Standard Ordnance Steel was cold rolled Decarbonized steel.
Winchester (shotgun barrel) Nickel Steel was likely AISI 2330. It is possibly that the Model 12s internal components were a higher carbon nickel steel.
Winchester Stainless Steel was probably Type 304 18/8
Winchester Proof Steel was likely AISI 4140

Send me a chunk of any of the above, and we'll know for sure wink