Underneath the Larger board showing the Route number and the main stops on the route London buses had a narrow slot which gave the final destination of this particular bus on the present journey. For example this might be the last trip of the day and instead of continuing to the very end of the route the bus would finish at the bus depot (in this case Catford Bus Garage). Anthony J Sargeant notes that remarkably the possible final destinations of every bus route working out of Catford Garage were written on a black roller blind (not just for, in this case, the No 54). Here you see the roller blind unwound and displayed outside the indicator board.
This meant that the driver for the front final destination board, and the conductor for the rear had to wind through this immense list of possible end destinations for every bus route working out of any particular London Transport bus garage to find the relevant one for each particular journey.