Check the arrangement on the ending theme music -- if the melody is played on a synthesizer, it's from season 6.
During the "Today on The Electric Company" bits at the beginning, there was a heavy drum/bass funk riff in the background and had the brass instruments playing the background riff while a synthesizer played a melody above it.
This is the only season underwriters' credits would crawl up the screen. The crawl, played to different re-recording of the show's theme, stated:
"Major funding for this program was provided by a grant from the U. S. Office of Education, Department of Health, Education and Welfare and by public television stations. Additional support was provided by unrestricted general program grants from the Corporation for Public Broadcasting and the Ford Foundation and by a grant from Carnegie Corporation of New York."