Susan Foreman's second TARDIS key

The First Doctor gave Susan a new TARDIS key as a replacement for the one he destroyed. (TV: The Sensorites)

After the First Doctor had been forced to give up his TARDIS key to on Destination, he ran off to find Susan knowing that she still had her key, hoping to beat the Master back to the ship. Though he found his granddaughter and her key, the Master managed to reach the TARDIS first. (AUDIO: The Destination Wars)

Later, knowing she would likely never see the TARDIS again, Susan intentionally abandoned the key in the dirt on the bank of the River Thames, after her grandfather helped her decide to start a new life in the 2160s by locking her out of the TARDIS. (TV: The Dalek Invasion of Earth)

Susan later recovered her key, and kept it with her in her new life on Earth. She knew that the Doctor's TARDIS was on its way whenever her key would glow. (AUDIO: Lucie Miller)