Without knowing its size, it's difficult to tell, but it looks like it could be one part of a 'sight' as used on mortars, or on machine guns used in the sustained fire role. This would be aligned with two or more marker posts, and elevation etc set by using the scale calibrated on the quadrant. The principle is not unlike a theodolite or sextant. It is very possible that, as already mentioned, it is off an artillery piece.