Anyone know what this is?

This cute little Chesterman (No.539) gauge is on eBay at the moment and I wondered what it was for? Not woodworking I guess. More of an engineering thing? Please post your thoughts if you know. Thanks.
Chesterman No.539_2
Chesterman No.539 _1
Chesterman No,539_4
Chesterman No.539_3