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δ³Σx² was inscribed on K9's regeneration unit. (TV: Regeneration) Similar inscriptions were seen by the First, Second and Third Doctors in the tomb of Rassilon. (TV: The Five Doctors)
Behind the scenes
In the wreckage of K9, writing on his regeneration unit resembles Old High Gallifreyan. (TV: Regeneration)
This is close to the Doctor's name "∂³Σx²" as given according to the 1972 Doctor Who reference guide The Making of Doctor Who, which is, however, not a valid source on this wiki.