In 1922 the newly independent Kingdom of Egypt encountered an immediate postage shortage. As an interim step (i.e., until new definitive stamps could be printed), the new postal authority opted to overprint select issues from the 1914 and 1921 "landmark" series (Scott 50 thru 74) with a black crown and the words "The Kingdom of Egypt" in Arabic.
Recently I learned there are, in fact, five different overprints. Four were applied in Cairo, one in London.
The Cairo overprints include one lithographed and three typographic. The London (typographic) overprint was applied by Harrison's.
“The Nile Post” (Egypt Specialized Catalogue) by Joseph H. Chalhoud includes several pages devoted to these overprints including an illustration of each overprint variety.