Anyone interested in seeing a collection of Mill/NMM boards I made some time back check out my Pinterest:

https://www.pinterest.com/mattlittlewood/merrill-boards/

There are some 200 odd images of Boards and designs based on the Nine Men's Morris concentric squares pattern.

The pattern is very old and was found as far back as excavations at Stonehenge. The symbol seems to have been used in the Pagan religion to ward off negative spirits.

It is interesting in the modern day in how many places we still find this pattern. In India we see it all over, from Bank logos to decorative designs. It is a symbol with meaning that even predates the game itself - which is already very old!