A mental picture of the envelope arose before him, the number, 30,
written upon it and the two black seals securing the lapels. He paused
again in his walk. His reflections had led him to a second definite
point and he fumbled in his waistcoat pocket for a time, seeking a
certain brass coin about the size of a halfpenny, having a square
hole in the middle and peculiar characters engraved around the
square, one on each of the four sides.

He failed to find the coin in his pocket, however, but he walked
briskly up a side street until he came to the entrance to a tube
station. Entering a public telephone call-box, he asked for the
number, City 400. Being put through and having deposited the necessary
fee in the box:

"Is that the Commissioner's Office, New Scotland Yard?" he asked.
"Yes! My name is Dr. Keppel Stuart. If Inspector Dunbar is there,
would you kindly allow me to speak to him."

There was a short interval, then:

"Hullo!" came--"is that Dr. Stuart?"

"Yes. That you, Inspector? I have just remembered something which I
should have observed in the first place if I had been really wide-awake.
The envelope--you know the one I mean?--the one bearing the number,
30, has been sealed with a Chinese coin, known as _cash._ I have just
recognized the fact and thought it wise to let you know at once."

"Are you sure?" asked Dunbar.