Argyll & Sutherland Highlanders on Norton 16H,most probably taken in the Middle East,number on front mudguard is in addition to the tank number is unusual.
The use of the O xxx or O xxxx registration is not so unusual as I have several pictures showing them.
The actual origine is unclear but it is usually shown on prewar manufactured machines used in the Middle East and North Africa.
Not known whether they always retained the C number as that is not always visible on the same picture, complicated by the fact it is only applied on the near side of the machines and many pictures are from the off side.