HMS Dreadnought British dreadnought battleship
HMS Dreadnought's forecastle deck, with guests aboard. One of whom is peering down 'A' turret's port 12-inch gun barrel.
Note the device on spotting top roof. This cylindrical object appears on a number of photographs of Dreadnought, mounted either on the roof or in front of the spotting top. The display of four zero's with the second digit part way around suggests it contains four drums numbered 0-9. In that case this might be a type of range clock used for displaying the range of a target to other ships in the battle line. In that case this particular device is unusual in being digital, whereas range clocks normally appeared as an analogue clock face.