The Oxford English Dictionary documentation of the word's first use is as follows: "rhotic ... 1968J. C. Wells in Progress Rep. Phonetics Lab. Univ. Coll. London (unpublished) June 56 It was possible to divide respondents into three categories: A. (non-rhotic) Those who had nonprevocalic r-colouring neither for ‑er nor for ‑a; B. (rhotic) Those who had nonprevocalic r-colouring for ‑er but not for ‑a; C. (hyperrhotic)." Cf. Wells's Twitter account at https://twitter.com/jcwells_phon/status/1136687808503062529.
John C Wells (1 December 1978) [Placed on the web 7 April 1999]. "Review of the Linguistic Atlas of England". The Times Higher Education Supplement – via UCL Psychology & Language Sciences.