The Five Gold Plates: A Puzzle

For the weekend, a puzzle (and my pastiche of a Sherlock Holmes story). The solution will be posted Monday.

A Puzzle

There was a knock on the door. "Ah, my dear Mrs Hudson. Do come in." said the great detective, lying languidly back in the armchair, and puffing pleasantly away at his pipe. Mrs Hudson came in. "A telegram, Mr Holmes." she said, "And it is marked urgent." "What's this?" cried Holmes. In one moment, all the placid calm of the man had gone, as, eyes . bright with excitement, he reached forward to grasp the telegram. It read: "Possession great worth + Five Gold Plates + In grave danger + Come at once + Tower of London at 5.00 p.m. + Urgent." "What can it mean?" asked Watson. "I do not know." replied the great man, "But we shall find out." Swiftly, he donned an Inverness cape: "Come, Watson, no time to lose. Grab your Bradshaw. The game is afoot. It is 4.30 now. We must hurry to the station."

When the intrepid pair reached the Tower, the London smog, foul and thick, had descended. There was a scream, sharp and piercing in the gloom. They raced forward, but they were too late. There, lying in a pool of blood, lay an American. "He needs medical care." remarked Watson, stooping forward. The man muttered something, and died. "Well, really I" said Watson, "I think he called me a moron." "I'll search his pockets for identification" said Holmes. "No need, old chap." remarked Watson, "his name is inside the book he dropped. Joseph Smith. They must have left this because they were only after the five gold plates." "Do you think so?" asked Holmes. "On the contrary, we now have enough clues to know that the thieves did not succeed in robbing him. Can you see the solution? It is, after all, quite elementary!"