This is a mass-produced, commercial replica of the Fob Watch as used by the tenth Doctor (David Tennant) in the new series. It is part of a larger device called the Chameleon Arch, which is able to hold the essence of a Time Lord, while creating a new false persona and memories, also changing his DNA to make for the ultimate disguise.
It was used in the episode "Human Nature" by the Doctor's companion Martha to protect him from the Family of Blood in 1913. It was also used to contain the Master's essence in "The Sound of Drums", in which episode the Master had taken on the false persona of Professor Yana. Measuring two inches in diameter, it is comprised of a zinc alloy casing and features a working time piece with an LED backlight.
