A leather shotfirer's belt with keys attached.
Shotfirers were employed in the mine to lay and detonate explosive charges to clear rock and break down the coal. The keys that they carried were used to fire detonators like the 'Demon' single shot exploder.
Shot exploders were used in mining because they could safely ignite an explosive charge without igniting any firedamp gas present in the mine atmosphere. The controlled explosions that they caused, however, were extremely dangerous, and could easily start secondary explosions of coal dust or gas.