fortis and lenis

[Frank Abate]

As linguistics was in its infancy in the 1930s, at least as a study under
that name, you might check under the heading of philology, or perhaps
historical linguistics.I would not be surprised if the (19th c.) German
Neogrammarians or their students used these terms in describing sounds in
ancient languages.  It is also possible that one of the ancient commentators
or grammarians used the terms.  Check in the Thesaurus Linguae Latinae for
evidence of the specialized senses.

Frank Abate