I must control the habit of replying to myself...
On second thoughts, and after more research, I now think this item is IoWSG, but in the design Kyoto and colourway Pine. The source of my insight is Mark Hill's little book, including the 'catalogues and shape' section at the back. There is a larger* such dish on p.101, although the image there looks more opaque than my specimen. It is said there that some items were given a slight iridescent finish, which can be seen on mine.
Trevor
* The item illustrated on p.101 is said be 24.5cm or 6.75in in diameter - which does not compute. Assuming the cm figure is correct, the data in the 'catalogues and shape' section suggests the item illustrated is 'New Kyoto'. That is also consistent with it having denser colouring than my, earlier, specimen (according to other information on that page).