The series was recorded on colour video, but some episodes only exist as b/w Telerecordings. This was a method to covert video to film, simply by pointing a film camera at a TV screen. These would be distributed to foreign broadcasters who did not have compatible video equipment, to use Telecine (video camera pointed at film projection) to make a broadcast tape in their format, or broadcast live from the film. Telerecording was also used to make recordings before video was available - We only have tv recording of Queen Elizabeth II coronation from Telerecordings made in this manner. The BBC pioneered recording video signals on to tape, first demonstrated in 1958.