Hole center gauge id and usage

[flathead4]

I found this "gauge" in my Dad's toolbox when he passed away in 2008. Can anyone tell me what it is and how it is used?

[old mart]

Its a mystery, are there any markings along the edges? The round boss at the end looks just like the hinge on a folding steel rule.

[flathead4]

No edge markings. The brass boss is fused to the gauge so don't think it could be a hinge.