If you look at the originals (select the thumbnail pictures) you will see that the head tablets of the stamps have been carefully removed and moved around between the stamps applied to the covers.

The earliest shows a green head tablet on a 2 piastre stamp with a blue head tablet on the joined pair. The 9 piastre stamp carries a red head tablet from one of the 2 piastre stamps above.
In the later cover the head tablets have been swapped between the 6 piastre and 1 piastre stamps.
James H. Mosey was the Principle of Kings School, and presumable a stamp collector, but who was his very clever fellow collector, or was he just a clever fraudster.?
Both covers are to the same address, the earliest is dated 27 MAY 48 and the later 20 AUG 48 the earlier is hand written and the later typed. There is no senders name and address. ( I wonder why?)
I have reason to believe there may well have been others similarly treated. It would be interesting to try and establish how many.
The quality of the work in removing the head tablets and moving them around between stamps is of a very high standard, with great care being taken to use the white band around the head tablet to mask any minor imperfections.
Careful examination suggests that the perpetrator probably made a punch and die to remove the head tablets, and used only the gum on the back of the tablet when inserting onto a cover. If the work was done by any other means it would have been almost impossible to have achieved the degree of interchangability, and tamper free appearance.
It is a surprise that covers of this nature got through the postal system without being picked up, since in effect they are fraudulent and should have been marked postage due, tampering with the Kings Head was after all an offence.
The fact they got through without being picked out by vigilant postal staff is a tribute to the quality of the work of the perpetrator, I for one would like to know how it was actually done, and who did it.