1970. First two digits.
Starting in 1973, they started the system of subtracting the first two digits from 100 (SN starting 26XXX= 1974 manufacture date)
Thanks all. Looks like mine was made in 1970. 45 years old and shoots perfect. 9 holes in a 50 cent size target at 25 yards shooting from a rest. That's about as good as I can do.