I will do so. I forgot what ship, but the ship just had numbers and since the cabins were only locked by stewards, I thought it would be embarrassing if a passenger was on the wrong deck and walked into the wrong cabin.
I think you're probably right, Lester, but if that image is (as I suspect) a computer graphic then we'd still need to see a photograph or artifact for definitive proof. The item shown below might be the genuine article, but if not it's probably a good representation.
What Bob posted is close to what the original looked like. The CGI image in the book is based on archival photos, but there is a photo of a D Deck cabin where you can see the stateroom number plaque above the door. It does say "D 25".
I think how they were numbered depended on the ship. I have seen some ships' accommodation plans (for 1st Class) where the cabins were numbered continuously without numbers being repeated on the various decks. In cases of Olympic/Titanic (in 1st and 2nd Class) each deck had a designation of course and the decks were marked by way of brass letters (or signs in 2nd Class) in areas where passengers would pass from one deck to the next.
When entering the corridors the cabins were also marked by deck, then number. I'll try post a picture if I can once I get home to illustrate the plaque.
In 3rd Class, to avoid confusion, numbers were not repeated and the cabins therefore likely just had the number plaque above them (even though each also belonged to a section).