As soon as I saw this letter, I knew it was the object of my search.
It’s true that its appearance was very different from the detailed description the Captain gave us.
On this one, the seal was large and black.
On the missing letter the seal was described as small and red with the royal crest.
The address on the letter I saw was written in small and feminine letters, while the missing letter was supposed to have an address written in large, bold letters.