Over the years, I've seen a few of these armbands featuring white "CD" letters on a black or very dark blue background. It appears these were issued in the Netherlands around the time of the country's liberation from German occupation. Some sources state they were issued by British forces but I cannot currently find a definitive source to corroborate this.
UPDATE: see the follow-up blog on an identical armband.
Send me items to blog about via my contact page