"A similar case concerns the Algerian postage stamp of 90c with the type "Rue De La Kasbah", printed in 1939, for single letters. But a tariff change of 1 December 1939, having increased the cost of these letters to 1F, rendered this 90c unusable for the function for which it was intended. But this time the copies of this stamp, initially very large print, were not destroyed, but overprinted 1F, by economy"
Scott does not give an issue date but notes that there are two overprint varieties. First was issued in 1939 and second was issued in 1940. Since the first is the last regular issue for 1939, that could be consistent with a Dec. 1 issue date.