"If you'll be good enough to lock that door, Postmaster," he directed, "I'll show you something else about your building that you didn't know. But I don't want anybody else coming in while we're discussing it."
Puzzled, but feeling that the government detective ought to be allowed to handle things in his own way, Rogers turned the key in the lock and came over to the table where Allison stood.
"Do you see that little square marked with a white star and the letter 'L'?" asked Elmer.
"Yes, what is it?"
"What is this large room next to it?" countered the operative.
"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."