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 ...
You seem to be correct. Or at least Medema (who is hopefully not you) agrees in a 2019 paper.
It was Francis Bator’s elaboration of “The Simple Analytics of Welfare Maximization” in
1957 that introduced the term “externality” into the economics literature (1957, 42, 43).
In contrast, Largreux (2010) says
the term ‘externality’ – which, apparently, ...