The 50 year old rule applies to the date of manufacture of the individual gun, not to the design date or the introduction date. Yes, a Model 70 made in 1958 is a C&R.
That is news and a surprise to me. I thought all C&Rs were listed specifically in the regs.
If what you say is correct (no offense, I'm a cautious doubter. allergic to steel bars.) many very "modern" type guns will soon be so designated. The Ted Williams shotgun I bought at Sears in 1955 would be a relic. My Ruger Single Six bought about 1970 will soon be, and etc.