The word factory comes to mind..

Think of a finished product made up of parts. Usually there is a base part as well. The base would be the stone balls. They are provided to value added developers to create a functional useful item. Maybe the stone balls were the basis for the large head statues found in the region. Or something else. The stone heads could be similar to large trophies commemorating a sports success.

The function and purpose as yet undetermined is the key.