Hi Tim,
the stone head seems to have little evidence of being 'worked'...which suggests that 'form' came before function. It appears that the club head was almost entirely formed by natural erosion by water and movement (rubbing against other rock fragments by water flow ...tides or fast flowing rivers for instance.)
Have you been able to ID the rock used ? Obviously this could help discover the possible region that it originated and the way it would have been worked.
There is the possibility that the head is very old, was 'lost' and subjected to natural erosion. Re-found, and as it had a 'natural hammer/club head' form was remounted on a shaft
The shaft seems to be 'mis-matched' with the head.....very egonomic with the slight curve...similar to late hatchet/small axe handles.
Flint as it is worked, flakes off leaving a facetted surface....however it is formed from molten lava and can form some very unusual organic shapes.....perhaps this club head was 'naturally formed and regarded by those that found it... to be a 'gift of the gods' and held in high esteem...it would also explain the lack of human intervention in its manufacture.