In the season two episode "The Gold Violin", the agency pitches a new campaign to a coffee client and featured the line "Now, we're aware that unsuccessful attempts have been made -- puppets and so forth -- to capture the attention of youth." Though not explicit, this is most likely a reference to Wilkins and Wontkins.
The season seven episode "The Forecast" features Joan's son watching the opening of Sesame Street. The "Sesame Street Theme" plays over dialogue throughout the rest of the scene.
While Joan speaks on the phone in the season seven episode "Person to Person", her son watches Ernie and Bert on a small kitchen TV.
John Aylward played Geoffrey Atherton in "Christmas Comes But Once a Year" and "Blowing Smoke" (2012)