The Gray Seal?
Oh, yes, they would know it was the Gray Seal--the insignia was familiar enough; familiar to the crooks of the underworld, who held it in awe; familiar to the police, to whom it was an added barb of ridicule.

He was placing it now, that insignia, a diamond-shaped, gray paper seal, on the panel of the door; and now, a black silk mask adjusted over his face, Jimmie Dale bent to insert the little steel instrument in the lock--a pitiful, paltry thing, a cheap lock, to fingers that could play so intimately with twirling knobs and dials, masters of the intricate mechanism of vaults and safes! And then, about to open the door, a sort of sudden dismay fell upon him.
He had not thought of that--somehow, it had not occurred to him! WHAT WAS IT THEY WERE WAITING FOR?
Why had they not struck at once, as, when he had first entered the house, he had supposed they would do?
What was it?
Why was it?
Was old Luddy out?
Were they waiting for his return--or what?
The door, without sound, moved gradually under his hand.

A faint odor assailed his nostrils! It was dark, very dark.

Across the room, in a direct line, was the doorway of the inner room--she had explained that in her letter.

It was slow progress to cross that room without sound, in silence--it was a snail's movement--for fear that even a muscle might crack.
And now he stood in the inner doorway.