This is from "Bombing Colours 1914-1937" by Bruce Robertson
In 1923 the stocks of Khaki were exhausted and from this time new finishes were applied. These were APD and APDT for home and overseas use respectively; the A for aluminium, PD for pigmented dope and T for tropical. These finishes bestowed the so called 'silver' finish of RAF aircraft between the wars. the effect of the tropical finish was the same as the home scheme, but the dope had different properties.
The book came out in 1972 you might be able to get it by ILL like I did many years ago. This is all it really has with this finish