The original photo is pretty big it it could very well be a O instead of a C. That O is obscured by shadow and it also for a part painted on the bracket of the Vokes filter.
Thanks Ron and Rik for the information on the KLG waterproof plug caps. I haven't got one, and dare say will never have one, but can you use them with modern plugs?
Modern plugs are metric and the KLG cap isn't I am afraid. I did manage to re-cut the thread in one of my caps but that was kind of scary to do but it worked out ok.