A Clue in Wax by Fred M. White

Cheriton's eyes lighted up expectantly. "Do you mean to say," he said when he got over his first feeling of astonishment, "that you are taking me to see the man who can put me on the track of--"

"Well, I didn't quite say that. It is pretty obvious to me, from what you have just said, that, whoever Lord Seagrane's murderer was, he must have lost the first joint of his right hand thumb. And, that being so he would see the danger of not having the damage repaired without loss of time. Of course he has not the slightest idea that you held the clue and that the piece of wax you gave me is going to be an important exhibit in the case. But I am going to take you to see a man named Harness, who is more than suspected of illicit operations, face-lifting and all that sort of thing, where certain criminals are concerned. You can see the importance of this to a man who wants an absolutely cast-iron disguise."