This is a tool for thickness calibrating, with good accuracy. It is mostly used for setting up any kind of tools having a depth stop, like the drill press or the thickness punch.
It is an ebony wedge, with its faces accurately dressed, and having scales engraved on top and side.
The measure ranges from 0.8 to 23 mm, with a resolution of 0.2 mm.
The engraved brass sheet is glued with epoxy, the wood is macassar ebony.
Here are a few pictures :