The first in-service full-featured FMS was on the A300, which went into service in 1974. Adoption snowballed with the introduction of the B767 and B757 in 1982 and 1983 respectively (though they'd both been in development since the mid-'70s). Therefore I picked the late '70s as a median, the point at which the industry committed to making the technology a part of the modern flight deck.