More discussion here:
These are now known as "Royal oblique misperfs". They are not considered errors, as they were intentionally done. And neither are they given catalogue listing.
The King kept several blocks (usually the plate number corner) for his collection, and traded the others with other collectors. King Fuad was inspired to do this to add interest and value to his stamp collection, and to have some desirable trading items to pass on to other collectors.
(The above illustrations of the Egyptian Royal Oblique misperfs are taken from the Feldman auction catalogue, 3 December 2018, which claims provenance of these blocks as being The Palace Collection of His Majesty, King Farouk.)
Now to get back to contemporary events. At a recent ICIS meeting via Zoom, one of the Heads of State mentioned this topic, and spoke glowingly of King Farouk's stamp collection. Another ruler inquired whether we could resurrect the practice, and all agreed to instruct the printers to prepare two sheets of each future issue as "Royal Oblique misperfs" for future Fifth World stamps.
The arrangement is that one of these mis-perfed sheets is presented to the King or President of the issuing country, and the other to the Distinguished Printer in appreciation for her or him having kindly co-operated to create these desirable errors.
Thank you, King Fuad, for a great suggestion!