It was later discovered that the the SM10 had luminous labels next to the jack sockets, so that it could be used in the dark; these things were highly radioactive and an urgent call went out from Blandford for all such items to be returned asap.
My guess is that it was radium, (Alpha radiation), that was used, which used to be used on watches, clocks, aircraft instruments and switches. This used to be painted on by hand by women. They were encouraged to straighten their paintbrushes with their teeth, and even used to paint the stuff on their teeth and face. The inevitable outcome wasn't pretty and resulted in a lawsuit against the company who employed them. All the management and scientists had taken considerable precautions to protect themselves from the effects of radiation, yet took no action to protect their employees.
Radium was replaced by tritium, (Beta radiation), which is still used in some watches, but many companies have replaced it with Superluminova, or Lumibrite for Seiko, Citizen etc.
The advantage of radioactive luminosity is that it does not require an external light source to activate it, it will glow by itself. Other lume requires to be exposed to an external light source to be activated, and depending on what type, and how the lume has been applied, will determine how long it will glow without further exposure to light.
PS. Sorry for the lesson in luminosity, horology is a bit of a hobby of mine.