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The sign accompanying the COAS. It reads

Apollo X (10) Lunar Module Optical Alignment Site

This Optical Alignment Sight is used to help astronauts align the maneuvering module with the passive module. It is a collimator-type device providing the astronaut with a fixed line-of-sight reference image. When viewed through the rendezvous window, the image appears to be the same distance away as the target.

The operator controls firing of the reaction control engines so as to keep the projected image superimposed on the passive module target. Range and rate of closure are determined by the size relationship between the image and the target. The adjustable light source projects the image from within its housing to the crewman's right eye by means of a beam splitter or combining glass.