"Bai Iear 15 or sou, it wud fainali bi posibl tu meik ius ov thi ridandant letez "c", "y" and "x" -- bai now jast a memori in the maindz ov ould doderez -- tu riplais "ch", "sh", and "th" rispektivli."

"By year 15 or so, it would finally be possible to make use of the redundant leters "c", "y", and "x" -- by now just a memory in the minds of old dodderers -- to replace "ch", "sh", and "th" respectively."

So "x" is removed as redundant, and then reintroduced for "th".