A.L. Dyke. Dyke's Automobile and Gasoline Engine Encyclopedia. Nineteenth Edition. Chicago: The Goodheart-Willcox Company, Inc., 1941.
According to Farm Collector:
McCormick-Deering 10-20 Track Layers (later named the TracTracTor Model 20) were built beginning in 1928. Production estimates based on monthly serial number data suggest that no more than 486 10-20 TracTracTors were built between Oct. 1, 1928 and May 1, 1930. Using the same measure, no more than 954 Model 20 TracTracTors were built between Aug. 15, 1930 and June 1, 1931.