In 1944, French Equatorial Africa overprinted some of its 1941 issues to produce a semi-postal set for which the excess non-postal funds were used to support the French Resistance (Scott #B22-35). Only 10,000 sets were issued, and Scott '12 prices the unused set at $252.00.
The set strongly appeals to French Colonies collectors, as well as World War II topicalists. Furthermore, since the former colony of French Equatorial Africa was granted independence and divided to form the current nations of Chad, Congo, Gabon, and Central African Republic, the set makes an interesting emerging market play as well.
A new feature, the Scarce Stamp Quantities Issued List, contains over 9,400 entries of stamps with printings of 100,000 or fewer. Take a look!
Also, those interested in learning more about investing in stamps may wish to view my new book, A Guide to Philatelic Investing (available on Kindle, but easily accessed w/any pc; digital list price = $5).