It’s a battery powered blackout light. It could be carried as a torch or clipped onto your bike.
Many children had bike lights like this in World War 2, including older children who volunteered as messengers, to carry important messages as part of Civil Defence.
They were also issued to Air Raid Wardens.
The lightbulb is hidden under a metal hood, which reflects the light onto the ground, so it can’t be seen by enemy planes.
Cars had similar light deflectors on their headlamps.
