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:

* 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/...

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)

I'm familiar with it and from UK but think it's mostly commonly used in academia and don't know where else it might be commonly seen.

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