Brown & Sharpe 559 surface square

I got this with a job lot of tools from a retired Toolmaker, there was lots of really neat stuff. At the bottom of the toolbox was this small old box and inside this unusually shaped beautifully made metrology tool. I wasn't sure what it was for at first but fortunately Mr Google was able to help and it is a four sided cylindrical square you use it in conjunction with a surface block standing on its end (as in the 1st photo). The measuring edge is 4"" long. The patent was taken out in 1957 and this looks like an early one by the age of the box, there is still a label on the top and you can just make out "Brown" and "559". The square also had "Sunbeam Toolroom" engraved on one side.