That does help a bit as Australia drives on the left side of the road hence mirror on the right side of the handlebar.Looks very much like an Australian registration label (and holder) to me. I guess the original photograph is not sharp enough to determine state/country of registration.
There was 6 of series B Meteors sent to Australia new all between September 1949 & Feb 1950 which is when Vincent transfers were been introduced for fuel tanks so this narrows it down a bit. The problem is the date of the photo is unknown so this could be bike that was later imported by an owner rather than by the factory.
Simon