[ New York, NY ]: Bell Telephone Laboratories, Inc. 1951 and 1955. First Separate Edition. Two items. Item One: Bell Telephone System Monograph 1841: "Single-frequency signaling system for long telephone trunks." 1-7,  (blank) pages. "Issued May, 1951" on inside rear cover. Some fading to grey covers, staple hole in front wrapper. Item Two: Bell Telephone System Monograph 2346: "In-band single-frequency signaling." 1-22,  (blank) pages. "Issued January, 1955" on inside rear cover. Both 8 3/8 x 10 7/8 inches, original blue and black printed grey paper wrappers stapled. 5 hole punched as issued. Otherwise very minor wear to extremities. Near Fine. Wraps. 
When local telephone exchanges evolved to support long distance calls, it was necessary to develop new protocols (electrically) to support these calls which were then routed through several telephone systems. In-band single frequency signaling was one such protocol - it allowed transmission of call related information to be passed from one telephone switch to the next to the next (and so on), as well as information necessary at the end of the call (person called hung up the phone, etc). In-band refers to sending these signals "in-band" meaning using the same phone line as the voice conversation was transmitted on. (later systems used "out of band" systems such as SMDI to transmit call related information but importantly not in the voice path).
The fact that the signaling information was on the same phone line as the voice transmission opened up the nascent long distance phone business to hackers (called Phone Phreakers) who were (in some cases) interested in learning about how the phone system was put together. Others, (black hats in today's parlance) were more interested in toll fraud - calling using the telephone system for free. In those days, long distance calls were expensive, and getting around "The Man" (the telephone company) was a game played much like that played today. This contest evolved because the telephone system providers, by definition, had to tell others how the signaling system worked or risk incompatibilities in the system.
There are two major papers which published the "keys to the kingdom" - In-band Signal frequency signaling [as offered here], and a later paper "signaling systems for control of telephone switching" .
We offer here the first "standard paper" published in 1954/1955, and a preceding paper published in 1951 which laid the foundations for that paper. Both are in the difficult to find first separate editions (some call them "offprints") of the Bell System Technical Journal articles (where they first appeared).