Seen on e-Bay, first time I came across one like it- is it what it is described to be or something else?
http://www.ebay.com/itm/WW2-DESPATCH-RIDER-Motorcycle-BLACKOUT-NIGHT-Lamp-COVER-Original-1942-/390643272501?pt=UK_Collectables_Militaria_LE&hash=item5af4274b35
the lens looks orange if it is it might be a cover you put over your headlight in foggy conditions the orange would penetrate the fog better than a standard headlight