Technical drawings. In the early 80’s I made many pen drawings with the well-known Rotring pens .25, .35 and .50 were the standards. It started out simple but eventually through cross-sections to the exploded views for the service diagrams. Straight lines were simple. The challenge always lay in the illogical curves and the conic objects.
Furthermore, there are also very old pen drawings on display, made with the dip pen, designs with letraset etc.