Oops - Yes I just noticed that it does have a tail fin in the centre ( so a triple fin ). However - reading up on it I learn that the last two of the four prototypes were indeed built with triple fins.
Those were LV633 and LV639. However it could not have been LV633 "Ascalon" ( Churchill's own transport ) as that one was fitted with square windows.
Is there anything else in that photo that would distinguish a prototype from a standard C Mk 1 apart from perhaps the unit code?