The page displayed are part of the Coronation series I have in my specialised collection of KG VI stamps. There are only two papers used, thick and thin, though the inks may differ.
The stamp you have is of the thick paper type (1938), in used condition it would have been worth around $10.00, but as it has a large hinge remnant and thinning of the paper as a result of the hinge being partly removed, the value would be around $5.00.
The paper is chalk surfaced and the colour is known as dull purple.
The watermark shows multiple crowns and C of A sideways (top of the crown facing left).
Perforation is 14 x 13½.
Quantity printed were 1,440,000 stamps.
The stamp below is of the type you have, this stamp is in superb mint unhinged condition and is valued around $45.