Continuity: When Peter Churchill is being flown back to France, he takes off in a Mk1 Halifax B, but over the drop zone he's in a Mk2. This may seem esoteric, but the Mk1 had in-line Rolls Royce Merlin engines, triangular fins and a nose turret similar to the Lancaster. The Mk2 had Bristol Hercules radial engines, square fins and a blunt nose with no gun turret, a radically different aircraft.