I often get people showing me stamps and asking if they are of any value. I can usually tell for certain that they are not, but I'd appreciate some feedback regarding this cover, which belongs to a friend of mine. Her father was a journalist covering a war in the Congo and got these stamps on the day of issue - they have a 'Premier Jour' (First Day) cancellation.

I know that Katanga was only independent for about two years and that stamps of Katanga on covers are quite scarce - please can anyone advise if collectors or dealers would be likely to be interested in this cover, and where would be the best place to sell it? Thanks!