Probably 1970s or maybe late '60s, when Case started using SS blades in some of their '65 pattern Folding Hunters. I think the majority of them were SS from mid/late '70s and later. Won't find many of the carbon steel versions past maybe mid 1970s or so. And from 1980s thru early 2000s or so, all or virtually all of them are in stainless steel. Case has started utilizing CV in some more recent versions of the 6265 though.
The collecting guide I refer to only shows the locking version of the 6165 in 1978 year of manufacture, specifically. Pattern# for that was 6165L SSP. The 'SSP' indicated stainless steel and a polished edge grind. I don't know if Case ever made that one in plain carbon/CV steel. If 1978 was the only year* the locking versions were made, I doubt they were carbon steel, as they'd transitioned away from carbon steel in most or all of the '65 pattern variations by then, I think.
Edited to add:
* - I do see some stamped with 3 dots for 1977, others into the early 1980s, and one 6165L SAB SSP from 1988 online. All appear to be in stainless steel.