Hi Bill-

Limiting this to the M70 and cataloged production, the short answer to the first question is 1936 in 220 SWIFT.  There are two barrel contours involved, the 26″ standard contour (#7) barrel used on standard rifles and Super Grades and the 26″ straight taper medium heavy contour (#8) used on the target model.  

Special order stainless barrels were available for many Winchester models for some time (mid-late 1920’s??).  These, of course, are easily identified by the baked on enamel (Japanned) finish used to blacken them (not to mention the fact that they are stamped “STAINLESS”):

However the cataloged production of stainless 220 SWIFT barrels began with the M54, in which the chambering was introduced in 1936.  At the time the 26″ Swift barrels were made of EITHER chrome moly steel or stainless.  I’d say that most first and second year production 220 SWIFT M70s (standard and target) that I’ve seen have the CMS barrel.  It’s easily identified b/c it have the same rust blued finish as the other pre-war CMS barrels:

However, the decision was made to drop the CMS barrels for 220 SWIFT in about mid-1937, so all barrels in that chambering dated after 1937 are stainless, and have the typical matte finish resulting from bead blasting and iron plating the barrels before bluing:

The use of stainless steel in cataloged M70 production is:

Barrel #7:  220 SWIFT (1936-1963) – Standard and SG; 264 WIN MAGNUM (1960) – Westerner.  

Barrel #8:  220 SWIFT (1936-1963) – Target model and Varmint; 243 WIN (1955-1963) – Target model and Varmint.

In the case of the Westerner, the stainless barrels were dropped during 1960 and only CMS used thereafter, so only the earlier Westerners have the matte stainless barrels.  These rifles usually have hand checkered stocks, while the later machine checkered ones mostly all have CMS barrels:

Identifying the late stainless M70 barrels can get tricky, however, because of the finish changes.  Consider the three Varmint rifles pictured below (all with stainless barrels). Prior to 1960, the barrels were bead blasted, iron plated, then blued:

In 1960, the factory adopted a direct black oxide bluing process to blacken stainless steel.  However the first big batch of barrels done this way were (unnecessarily) iron plated before bluing (albeit not bead blasted).  The result looks exactly like the CMS barrels.  Unlike the matte barrels, the muzzle face of these is blackened b/c even though the muzzle was protected during plating, the finish process “took” on the bare stainless steel:

The final barrels (affecting 220 SWIFT Varmint rifles and 243 WIN Varmint and Target rifles) were finished without iron plating, so the finish is polished but slightly “off” in color compared to the regular CMS barrels of the day:

That about covers it for cataloged M70 production as far as I know…  However stainless barrels were available on special order in other calibers/contours.  I’ve seen stainless barrels in 270 WIN (24″ standard), 300 H&H MAGNUM (26″ standard), and 30-06 (24″ medium heavy), and I’m sure there were many others made in small numbers. 

As for the metallurgy…  Rule’s book states that two different stainless alloys were used over the course of production, but does not go into detail.  Maybe seewin can help with that one, but I cannot…

Hope this helps!!!  And isn’t too much overkill!!! 

Lou