A Type 50 TARDIS was the TARDIS that the First Doctor used prior to his life as a renegade Time Lord. (AUDIO: Prisoners of Fate) The Doctor decided to steal another TARDIS, a Type 40, when he finally departed Gallifrey with his granddaughter Susan Foreman. (TV: The Name of the Doctor)
The Type 50 TARDIS later escaped Gallifrey on its own and travelled to Valderon, where the Doctor encountered it during his fifth incarnation in 3556. (AUDIO: Prisoners of Fate)