It’s a Weidenhof made tester, not that old as early testers were 2 volt, one cell at a time.
This one is for the full 12V as modern batteries have inaccessible internal cell connectors.
I have this model and it’s big brother, it has another resistance grid for testing at double the load if required.
Undoing the thumbscrew opens the shunt and allows no load voltage to be read.