This picture shows the traffic light Gutter pair Jim1952 refers to. Harrison printed the traffic light in the sheet margin. Harrison's Jumelle press printed double pane sheets separated by a central gutter. For the period I mentioned above, Harrison printed the traffic light in the central guyyer margin, giving rise to "Traffic Light Gutter Pairs." Without the traffic light, it is just a "Gutter Pair."
In some issues, the traffic light extends beyond the gutter of the second column.