This helmet was submitted to the website by the owner. It is an unusual marking with the letters D and R either side of what appears to be an ARP decal. It is assumed the letter are from Despatch Rider (sometime spelt Dispatch Rider). The owner queries whether it could be Demolition & Rescue but that term has never appeared in any of the documents I have seen.
News about interesting insignia, ARP related info and period photos that turn up.