This exhibit is on display in the cellars of the really quite marvellous Museum of Warsaw. I confess to not having heard of this game before, but it has been played since as early as the Roman period, and appears to have been relatively common. This particular board is made from clay and was discovered during excavations of the Royal Castle in 1971 and it’s thought to date from the seventeenth century.
For anyone who wants to know the history of the game, there’s more information at https://en.wikipedia.org/wiki/Nine_men%27s_morris. What fascinated me about this game is that they’ve discovered it etched into the cloister seats at Norwich Cathedral, I assume by some youngsters who wanted to amuse themselves during some less than interesting hours sitting and singing. As an aside, it seems that if two experts play the game then it always ends up in a draw, which isn’t an ideal situation.