Found in dads tool box.

I spotted this item in one of my dads old tool boxes. I could not even begin to guess what it was used for so he put me out of my misery. It is used to measure an indentation left by a metal ball during a test which is carried out to find a materials hardness. The Brinell hardness test was proposed by Johan August Brinell in 1900. In this test a hardened steel or carbide ball of a known diameter is pushed into the surface of the material being tested, with a known load. The diameter of the indentation in the material surface is an indication of the Brinell Hardness Number. The diameter of the indentation can be measured by the small scale which is visable at the base of this instrument by the magnifying glass.
I have tried searching this instrument on the internet but all I can find is information about the more modern testing machines which are used today. However I find this one much more interesting, so if there are any engineers which may stumble upon this entry that have any more information it would be gladly recieved.