The first season of Sesame Street aired in 1969-1970 and this photo of the cast dates from then:
The giveaway is Oscar the Grouch's orange fur...it would switch to green for the second season. Here's the first 15 minutes of the first episode:
And here's part 2, part 3, and part 4.
Sesame Street is insane, BTW. They aired 130 60-minute episodes over six months for that first season and over its 43 seasons, the show has averaged 100 episodes per season. A truly amazing combination of quantity and quality.