"That's the--why, that's the registry room!" "Precisely. And concealed in the wall in a spot known only to persons familiar with this blue print, is a tiny closet, or 'lookout.' That's what the 'L' means and that's the reason that there's a strict rule about guarding plans of this nature very carefully." "You mean to say that a place has been provided for supervision of the registry division--a room from which the clerks can be watched without their knowledge?" "Exactly--and such a precaution has been taken in practically every post office of any size in the country. Only the older men in the Service know about it, which is the reason that neither Boyd nor any of his men asked to see this set of plans. The next step is to find the key to the lookout and start in on a very monotonous spell of watchful waiting. You have the bunch of master keys, of course?" "Yes, they're in the safe where the plans were kept. Just a moment and I'll get them." When Rogers produced the collection of keys, Allison ran hurriedly over them and selected one which bore, on the handle, a small six-pointed star corresponding to the mark on the blue print. "Want to go up with me and investigate the secret chamber?" he inquired. "I certainly do," agreed Rogers. "But there's one point where this room won't help us in the slightest. How did the thief get the mail containing the money out of the building? You know the system that maintains in the registry room? It's practically impossible for a sheet of paper to be taken out of there, particularly when we are on guard, as we are now." "That's true," Allison admitted, "but it's been my experience that problems which appear the most puzzling are, after all, the simplest of explanation. You remember the Philadelphia mint robbery--the one that Drummond solved in less than six hours? This may prove to be just as easy." There Allison was wrong, dead wrong--as he had to admit some ten days later, when, worn with the strain of sitting for hours at a time with his eyes glued to the ventilator which masked the opening to the lookout, he finally came to the conclusion that something would have to be done to speed things up. It was true that no new robberies had occurred in the meantime, but neither had any of the old ones been punished. The lost one hundred thousand dollars was still lost; though the department, with the aid of the Treasury officials, had seen that the banks were reimbursed. "The decoy letter," thought Allison, "is probably the oldest dodge in the world. But, who knows, it may work again in this case--provided we stage-manage it sufficiently carefully."