I understand a lot of the bikes that went to the US were in a largish batch that went from auction. I've not seen B2 on the few photos I've seen of the others so no clues, to me, there. But I would guess it as some kind of section i.d. marking. The remains of the Union Flag on the other side is just typical of the flag marking applied to vehicles post war, such as in BAOR. Maybe some 'old hand' on here might come up with more?
email (option): dncnsimpsonAT the well known 'warm'mail.com