Do you know what this is?
It’s a one of these! A map measurer. In the ‘good old days’ before we used the Internet for everything it was a brilliant device for measuring how far a walk or drive was on a map. You roiled it along the route from start to finish and then rolled it back along the scale bar to see what the distance was. Much more accurate than trying to do it with a ruler because you could follow all the ins-and-outs of the road or path. (The map in the background is the old-fashioned 1 mile to 1 inch scale that was the mainstay of the Ordnance Survey for many decades.)
