Hi Mairiuna ! I doubt you have been able to find the solution to the stamp mystery, since you would need a magnifying glass and the physical copies of the stamps to find the hidden letters.
That’s why I will give you the solution right now!
The letter D is found on The Hound of the Baskervilles stamp, at the bottom center.
The letter O, on The Final Problem stamp. It’s the cuff-link
The letter Y, on The Greek Interpreter one, under his knee.
The letter L, on The Six Napoleons stamp. On the table, there’s a book. It’s on the cover.
The letter E, on The Reigate Squire, near Watson’s right shoulder. Examine the spine of the cover.
The above pictures were taken on a superb site devoted to detective fiction on stamps. And you know what? Sherlock Holmes has a complete page of it’s own! A must see for all sherlockians.
Here’s the link : http://www.trussel.com/detfic/sholmes.htm
What a superb topical stamp collection !