I've seen it used somewhat often in the UK (almost entirely in academia as far as I can recall), at least frequently enough that I've used it in my own writings despite not studying history.
At a quick search, I can find a couple of sources that suggest it for notetaking at least:
FWIW I'm from the UK and was taught this notation in high school history classes (though the C encircled the 19 when hand-written).
That said I had trouble parsing the "C19" too so I don't think I've seen it used in print much. I tried to read it as a UK postcode zone first :) (I used to live in SW19)
UK. Wikipedia doesn't have an explicit explanation/history of it that I can find, but it does feature on disambiguation pages, e.g. https://en.wikipedia.org/wiki/C17
At a quick search, I can find a couple of sources that suggest it for notetaking at least:
* University of Portsmouth (listed under "Common general abbreviations") https://www.port.ac.uk/student-life/help-and-advice/study-sk...
* University of South Australia (under "Common symbols and abbreviations") https://lo.unisa.edu.au/pluginfile.php/1687781/mod_resource/...