This is one of the screen-used tracers from the Doctor Who season 16 "Key to Time" storyline (1978-1979). It was originally obtained from Peter Pegrum (head of the visual FX department in London) and sold at the Visions convention in December 1997. Four are believed to have been made. One was wired to light up and another was broken at the end of "The Armageddon Factor." The three Key to Time pieces pictured with it are also authentic.
