I love this circuit diagram for a laboratory model pH meter. It was obviously held in a binder at some point (and perhaps mishandled a bit, given the conditions of the holes). But the markings are the best part. 

Some portions of the diagram have been traced with red (indicating first position) and some with blue (indicating second position). This addition doesn’t just tell us about how the circuit worked—it tells us how people used the diagram. We are given insight into the life of this specific object.