The following is a timeline for the development of the limiterless FSK demodulator.
1918 (December) - U. S. Patent 1,286,351 issued to Howard Krum of the Morkrum Company on the start-stop system.
1948 (June) - U. S. Patent 2,443,434 issued to Robert Sprague of Press Wireless, Inc. on the ATC circuit.
1953 (January) - RTTY bulletin published and edited by Merrill Swan, W6AEE.
1954 (October) - William M. Gates "This is a Terminal Unit - I Am a Ham." RTTY Bulletin.
This is one of the earliest amateur article to describe a demodulator that lacks a limiter. It includes an ATC-like circuit.
1957 (March) - J. W. Allnatt, E. D. J. Jones and H. B. Law published their findings on limiterless demodulation in the Proceedings of the IEE.
1961 (September) - U. S. Patent 2,999,925 issued to Elmer Thomas of Page Communications, Inc on the DTC circuit.
1963 (June) - Frank Gaudé, K6IBE "A New Approach to TU Design Using a Limiterless Two-Tone Method." RTTY Bulletin.
This is the first amateur article that methodically describes the use of a true ATC circuit ("slideback detector").
1963 (September) - R. H. Weitbrecht, W6NRM "What Is This Two-Tone Detector?" RTTY Bulletin.
This is the first amateur article that highlights the frequency diversity property of limiterless demodulation.
1964 (January) - Victor Poor, K3NIO "A Second Look At Limiterless FSK Detection." RTTY Bulletin.
This is the article that introduces the amateur to the Page Wireless DTC patent.
1964 (November) - Irvin Hoff, K8DKC and Keith Petersen W8SDZ "The Mainline TT/L FSK Demodulator." RTTY Bulletin.
This is a highly optimize FSK demodulator that incorporates most of the advances in the 1963-1964 time frame.