Mapperley produced from his breast pocket some papers, and from amongst them selected an envelope—the azure-tinted envelope which he had picked up from the caretaker's supper table at St. Mary's Mansions.
"You recognise this?" he said, with a sly smile. "You know where I got it. This is the envelope which Baseverie took to the caretaker, with the order to enter Madame Listorelle's flat. You knew that I carried it off, from under the man's nose, last night. But you didn't know why. I only laughed when you asked me."
"Well, why, then?" inquired Hetherwick.
"This reason," replied Mapperley. "We both noticed that the sheet of paper on which the order had been written by Madame had been shortened—there was no doubt that a printed or embossed address had been trimmed off, rather roughly, too. We noticed that, I say, both of us. But I don't think you noticed something far more important—far, far more important—for our purposes."
"No," admitted Hetherwick. "I didn't. What?"
"This," said Mapperley, turning back the broken flap of the envelope. "You didn't notice that here, on the envelope, is the name and address of the stationer who supplied this stuff! There you are—W. H. Calkin, 85, Broadway, Westminster. You never saw that, Mr. Hetherwick. But I did!"
Hetherwick began to comprehend. He smiled—gratefully.
"Smart of you, Mapperley!" he exclaimed. "I see! And—you've been there?"
"I've been there," answered Mapperley. "I saw a chance of tracking these men down. I couldn't get hold of Calkin till nearly noon, but I got on like a house afire when I did get him. You see," he went on, "that paper is, to start with, of an unusual tint, in colour. Secondly, it's of very superior quality, though very thin—intended chiefly for foreign correspondence. Thirdly, it's expensive. Now, I felt certain its use would be limited, and what I wanted to find out from the stationer was—to whom he'd supplied it. That was easy. He recognised the paper and envelope at once. Of the handwriting on the paper, he knew nothing whatever—Madame's writing, you know—that he'd never seen before. But he said at once that he'd only supplied that particular make of paper and envelopes to three people, and for each person he'd prepared a die, to emboss the addresses. The embossing had been done at his shop, and he showed me specimens of each. One was for the Dowager Lady Markentree, 120, Grosvenor Gardens. That was no use. The second was for Miss Chelandry, 87, Ebury Street. That was out of count, too. But the third was what I wanted. It was just the address, 56, Little Smith Street, S.W.1. As soon as I saw it, I knew I'd got on the right track."
"Go on!" said Hetherwick.