Thanks for the interest Peter. Here are a few other pictures which might help with answers.
The first is a brass example of the same medal showing machine marks.


Queen Charlotte Death Medals 1818 RRR. BHM #968
Obv. Diademed head of Queen Charlotte, right. H . M . G . M . QUEEN CHARLOTTE.
Rev. BORN MAY 19 . 1744 / MARRIED / SEPR. 9 . 1761 . / DIED AT KEW / NOVR. 17 . 1818 / AFTER A VIRTUOUS / REIGN WITH HER / BELOVED KING / OF 57 YEARS
AE, AE silvered 25 by ? AE RRR; AE silvered RRR. Unlisted in Brass.
Shows that the copying machine could not engraver the centre of the medal, looks like the 'U' in Queen might have lines, that might be wishful thinking but not when you see the following picture.

This picture shows some hand-engraving done to the centre of the die after the the above medal was struck and also some of the copy/reducing lines have been removed by some sort of polishing......
.but most importantly shows very clear lines across the letters...., I assume the medal above it has had the lines on the letters worn down by handling.
Now there is some doubt as to whether this is by Kettle or an unknown medalist but seeing as all the other Kettle medals used hand punched letters into the Victorian era I now think it was issued by Kempson & your idea that it was a trade secret that he carried to his grave could be correct. The fact that Kempson issued medals for Princess Charlotte's death in 1817 & Kettle did not, that Kempson issued medals for Queen Charlotte's death 1818 and that both these medals have lines across the letters and I have found no medal that is definitely by Kettle with these lines seems too much of a coincidence. I need to put my thinking cap on!
Comparing the letter in 'Charlotte' they look to be the same style so it appears even more likely they were both issued by Kempson.