From Wiki

Blundell Rules Limited Radiac Calculator No. 1

• Type: Circular Custom
• Size: 5 inches / 13 cm diameter
• Material: Celluloid
• Scales:
  • Dose, Dose rate / Time of current reading / Time of future reading (Rotor disks are reversible for land and sea propagation)
• HvH: 566

Properly, the "Calculators, Radiac No 1, Z8/6665-99-911-0117". Calculators in the plural as the unit includes reversible rotor discs to account for the differences in radiation propagation on land and asea. Set the time since the initial blast against the current radiation measurement, then set the time (again measured since the blast) you entered the contaminated area. Then you can read the current expected radiation level and how much you absorbed over the interval you were exposed. In rubberized (er, "rubberised") cloth case.

Obtained from a charity auction hosted by Slide Rule Universe.

The lot also included a patch and official armband from the British Civil Defence Corps: