The Puzzle Lock

"He ran his finger down the list of names and had just turned to the account of one of the customers when we were startled by a loud click from the direction of the strong-room. We both turned sharply and beheld Thorndyke grasping the handle of the strong-room door, and I saw with amazement that the door was now slightly ajar. "God!" exclaimed Miller, shutting the ledger and starting forward, "he's got it open!" He strode over to the door, and directing an eager look at the indicator of the lock, burst into a laugh. "Well, I'm hanged!" he exclaimed. "Why, it was unlocked all the time! To think that none of us had the sense to tug the handle! But isn't it just like old Luttrell to have a fool's answer like that to the blessed puzzle!" I looked at the indicator, not a little astonished to observe the row of fifteen A's, which apparently formed the key combination. It may have been a very amusing joke on Mr. Luttrell's part, but it did not look very secure. Thorndyke regarded us with an inscrutable glance and still grasped the handle, holding the door a bare half-inch open."