the last portion of the flight path more vertical. The trail adjustment, set by dialling in the measured terminal velocity for the bombs being dropped, used a cam to move the height bar forward away from the vertical, reducing the range angle and thereby reducing the range to account for this effect. Many thousands of CSBSs were sold around the world, and many other sights were developed from the basic idea. In the mid-1930s, the basic CSBS concept was largely universal for production bombsights. Mk. VII and IX During its development prior to the opening of World War II, the CSBS added several new features. A simple modification found on pre-war models was the Auxiliary Drift Bar attachment. This consisted of a single drift wire in a C-shaped clamp that could be moved along the main drift wires, and rotated in relation to them. Previously, the bomb aimer would use the main drift bar as a tool to measure wind speed, but it was found that the bomb aimers would forget to reset it to the proper angle for bombing when things got busy. These same measurements could be made with the Auxiliary Bar, leaving the main drift bar in the proper position. Later versions used by RAF Coastal Command and the Royal Navy also included a further adjustment, the Fourth Vector, for attacking moving targets. This was primarily intended for use against ships and submarines. This was a fairly complex system of rotating rings and sliders that allowed the bomb aimer to dial in the relative course of the target and its estimated speed. This moved the backsight directly fore and aft, and turning the heading dial adjusted how much the speed dial moved the backsight. As the resulting mechanism was fairly large and complex, the sights were also available with the Fourth Vector removed, denoted with a *, as in the Mk. IX A*. Mk. X Prior to the war a major redesign of the CSBS was underway. The new Mk. X replaced the vertical slider used for altitude adjustment with a horizontally-moving backsight at the top of the device, and the entire foresight and drift wire area was made considerably smaller. The calculator and wind drift settings, formerly mounted on top and in front of the large compass at the rear of the earlier models, was moved to the left side of the device and changed in