It was never broadcast on the actual show, it was specially taped as an in-joke. This is the story from Wikipedia:
In 1979, the cast and crew of Rainbow made a special edition for the Thames TV staff Christmas tape, sometimes referred to as the "Twangers" episode. This show featured plenty of intended sexual innuendo (beginning with Zippy peeling a banana, saying "One skin, two skin, three skin, four..." before being interrupted), and although not shown at the time (as it was never intended to air), it has since been aired for the amusement of those who grew up with the show. The clip became famous after being aired on Victor Lewis-Smith's Channel 4 programme TV Offal (1997) and was jokingly referred to as 'the pilot episode', which it clearly wasn't as Geoffrey Hayes was presenter at the time. (The external page, linked below, erroneously claims that the episode was publicly broadcast as a regular episode.)[1]
