Note the detail on the backwards numbers and the ability to go beyond 3 minutes to accomadate all manner of timings.
The Good Egg Timer of Memory was a Fifth Inspector device, retaining her memory of the previous 10 minutes of events at all times, except, of course, when it didn't work, and instead retained the 10 minutes yet to come. Originally, the device was created for use in the the serial Twenty Or So Minutes 'till Four, but rewrites to the "too confusing" script eliminated the need for it as a device.
Solving a Problem[]
While unused, the device had been shown on screen, and for length reasons, the filmed clips with the Inspector checking the device upon occasion were kept. Screenwriter Michelle Ticks, in the closing story of the Fifth Inspector's run, decided to refer back to these clips, and use the device to bridge the final moments of the Fifth Inspector with the first moments of the Sixth. This enabled Graham Chapman's Inspector to retain the operating instructions to the Lockpick of Space, as well as knowledge of her/his Associates.