The letters are for the Royal Corps of Invalids.
(not Royal Chelsea Infirmary or Royal Chelsea Invalids)
The buttons are worn by the Chelsea Pensioners, living at the Royal Hospital Chelsea.
links:
https://en.wikipedia.org/wiki/Royal_Hospital_Chelsea
https://en.wikipedia.org/wiki/Chelsea_Pensioner
This particular button is post 1952, made during the reign of Queen Elisabeth II