Motorist's Map Measurer
Practical Motorist was a popular magazine in the 1930ís. it was a DIY magazine to enable the ordinary motorist solve a variety of common mechanical issues. Publication was suspended during WW2. But was reintroduced in the 1950s.
In those days the motorist relied on printed maps. (No internet in those days) By running the map measurer across the roads, the motorist could determine the distance between different points. One side was in kilometres the other in miles. The map measurer had a magnifying glass to assist.