Solution

It's a coconut grating stool! A schoolboy made it for me on the island of Rotuma in the South Pacific back in 1973. It was carved from one solid piece of wood before a rudimentary grating blade was attached and held in place with a piece of scrap tin.

The idea is that you straddle the stool then take halves of mature coconuts and between your legs grate away at the white flesh within. A bowl would be positioned beneath the blade to catch the grated flesh.

Later the grated coconut would be pressed or squeezed to capture the milkiness contained in the flesh. This oily coconut milk was very useful in cooking. The remaining gratings would be fed to chickens.

It seems that pretty much everywhere that coconut palms grow naturally - from eastern Africa to India and the Pacific islands - human beings came up with similar devices. There are many variations upon this theme and some of them could appear far more stylish than my rustic device.

The grating stool shown below was made in the nineteenth century on Nukuoro Island in the area of The Pacific known as Micronesia. Here, rather than a metal grating tool there is a hard piece of seashell secured by coconut fibre twine.

The picture below shows a large coconut grinding stool at Noa'tau on the very  same island, north of the Fiji Islands,  where my prized device was made. I believe that the photo was taken in the 1920's:-

©Alexander Turnbull Library, Wellington, New Zealand