As promised...
A (very brief) history of the Textus Receptus: (with thanks to Dr. James White and his The King James Only Controversy)
Desiderius Erasmus published the first printed Greek edition of the New Testament in 1516. He translated it from a handful of manuscripts that were available to him at the time. In 1519 Erasmus' second edition was printed, followed by further editions followed in 1522, 1527, and 1535. Incidentally, none of the manuscripts available to Erasmus had the complete text of Revelation.
Robert Estienne (aka Stephanus) took Erasmus' work further, adding marginal notes taken from another printed work. He printed four editions, the third of which (1550) "was very popular in England" (p. 105).
Theodore Beza printed nine editions of the NT in Greek, adding further to the collection of variant readings. Notably, he made changes to the text that are not supported by manuscript evidence at all.
The term "Textus Receptus" itself was not applied to this family of texts until well after the publication of the KJV, when in 1633 the Elzevir brothers released their own second edition. The name stuck, though as White notes, "Even the standard Textus Receptus use today... is not identical to Erasmus, Stephanus, or Beza but is instead an 'eclectic' text that draws from various sources." (p. 106).