Need Help With Guinea C24 Through C26 Overprints

According to the 2014 Scott Catalog, the overprint colors for C24 and C26 is carmine or orange. The overprint color for C25 is blue. A black overprint for C24 or C26 means the stamp came from a souvenir gift booklet celebrating Guinea's 3rd anniversary of independence. The bookle was sold at the NY World's Fair, but the stamps aren't valid for postage.

As you can see on my uploaded stamp image, the overprint on my C24 is green. Does anyone know whether or not it's valid for postage? If not, then what is it? A cinderella?

The first thing you need to do is get a copy of the booklet and see if what the catalog says is correct . My guess is the three overprints that you show above is what is in the booklet and the catalog is being lazy and say the black is on all three values when in fact it is only on the 500f stamp.

Thank you, Floortrader! Any idea how I might go about finding that souvenir booklet?