In Raymond Williams's Keywords (pdf here), one can read:
Culture as an independent noun, an abstract process or the product of such a process, is not important before 1C18 and is not common before mC19.
Williams, R. (2014). Keywords: A vocabulary of culture and society. Oxford University Press. p. 88.
I have never encountered this use of date format, does anyone know what dates "1C18", and "mC19" refer to? Other occurrences include "eC19", "C1S" or "C18". I thought of a misprint in the first place, but this is being used consistently over multiple editions.
After some thinking, I came to the idea that these refer to centuries: "C18" as in the 18th century; "eC19" as in the early 19th century, but I could not find any references to support it, nor can I understand some formats, like "C1S", or "1C18".