^^ The dynamo is on the front wheel- isn't it? There is a generator mounted on the rear as well.
The square thing looks like a heatsink and part of the mechanism for the front brake.
I doubt the "B" is for 'Balloon' unless the Brits had a completely different idea of what balloon tires were all about. Balloon tires were usually wider, but I am an American so balloon tires may have a special meaning here.