I suspect you have found the author responsible for the original coinage of the word "externality" within economics, with this meaning.
In the 1950s (and earlier), the discussion of externalities was typically done using the phrase "external economies", as Francis M. Bator frequently did too. Bator built on Scitovsky (1954) and Meade (1952), both of whom used "external economies", as did JK Galbraith and others around that time (as well as several others before them).
Boudreaux and Meiners wrote the paper: "Externality: Origins and Classifications", 2019, within which they say:
Bator's article likely forms the basis for the now-common notion of
which is probably as close as we can get to confirmation that the word is Bator's original coinage for this particular meaning, together with Fizz's Medema reference. There's a long footnote in the reference that Fizz found - Medema 2019 (p20) (my emphasis):
Bator first used the term in his 1956 MIT Ph.D. thesis, from which his 1957 and 1958 articles were derived (Bator 1956). Whether it was Bator who coined the term is hard to say. Paul Samuelson, Bator’s MIT colleague, used it at nearly the same time in an article on the subject of intertemporal price equilibrium that appeared in Weltwirtschaftliches Archiv(Samuelson 1957). Samuelson noted simply that “knowledge is a resource loaded with externality” (1957, 210). Bator’s article seems to have appeared first and was certainly more widely read, being cited nearly fifty times over the next dozen years as against eighteen cites to Samuelson’s piece. (Source: Google Scholar, accessed August 22, 2017). Moreover, none of the citations to Samuelson’s article were in the welfare economics/externalities context. There is no input from Samuelson acknowledged in Bator’s opening footnote, nor is mention of Bator made in Samuelson’s article. The term was picked up relatively quickly though, being used another half-dozen times before 1960. It was also Bator who introduced the term “market failure” into the literature, this in his 1958 article, “The Anatomy of Market Failure.
Bator's 1956 PhD thesis for MIT (pdf) uses the term 78 times, with no earlier citations or quotes of its use.
So, with both Samuelson (1957) and Bator writing around that time about externalities, using that specific word, and both at MIT, there's a fair chance that the word was coined there within that MIT department by one of them, or a colleague of theirs (with Robert Solow being the most likely candidate, as Bator's PhD supervisor), and entered the writings of both of them organically at around the same time, through the usual social and professional interactions that go on in an academic department.
And Medema's footnote quoted above makes a persuasive case that it is Bator's 1957 & 1958 papers that established the term in the literature, from where it became widely disseminated.
Robert Solow is, at time of writing, still with us. I wonder if a direct enquiry to him might shed any light on this?