That is indeed a rare Humber, so good to see another one exists, there were rumours of another Humber convertible thought to exist abroad somewhere.
They were a coachbuilt body made only for military use, there are the Two famous ex-Field Marshall Montgomery Humber covertibles in existance that he used throughout WW2, one is on display in the Coventry Transport museum, the other one is at the Imperial War museum Duxford.
The only other known Military Humber covertible in the UK belongs to the "History On Wheels Museum" at Eton Wick near Windsor, Berkshire, not far from London, contact Peter Oliver they would be very interested to hear of another Humber convertible, the museum opens once a month, next open day is sunday June 26th
http://www.historyonwheels.co.uk/
There is another Humber convertible that appears at shows and parades but it is actually a re creation based on a converted miltary Humber saloon staff car