Thanks for the comments so far.
Jonny, I'm pretty sure it's not home made, there's another exactly like it on ebay uk at the moment. If I'm right in thinking they were given to surveyors then I think it's probably for measuring something that might wear. I'm still curious about the first two.
On the other hand I'm totally convinced by 90LX_Notch. Thank you for identifying it. At least now I can throw it out without later discovering it's vital for some other piece of gear I have. It remains a mystery (and probably always will) how my father came by it. To the best of my knowledge he never touched an exercise machine in his life.
Russell