This little wooden box was suspended by rubber cords either between the struts or, somewhat more aerodynamically efficient behind the seat. It consists of a watch movement turning the little drum upon which a strip of paper with altitude lines was affixed. A little ink writer linked to an altimeter would then record the altitude at any given point of time during the flight. The pilot would then annotate the strip upon his return with the events of his mission. It also allowed commanding officers to verify altitudes flown at any given stage of the mission. Simple but very effective.