This tool would appears to be a drilling jig and our best guess is that it's for Midland Railway carriage sides. That rectangle of holes would be drilled and then joined up to form a round cornered window. More to the point, you could form a series of such windows that were accurate and identical.
The other holes are still a mystery and the guess above might be wide of the mark.
One of our members has claimed this as he thinks he has a use for it. As an engineer, he recognises the quality of the device so at least one has a good home.