this is all I can find
https://www.ukbusinessforums.co.uk/threads/post-nominal-letters-are-they-regulated.99792/
There WAS a case where a non colonel MC landed up in court and limply said MC stood for Master Comptroller.
Cant remember what happened
Oh hey yeah
when MC was invented around 1915
The letters ( or - - - post nominals) MC were already in use for Master of Surgery ( or Chirurgery ) at Cambridge
So they wrote to all the old buggas and asked them to change in er 1916.
and all the old buggas trooped up to senate house and voted..... no ! so they kept MC
The last cambridge master of surgery MC died aged 115 and 1/2 a few years ago.....
( so it was changed to M Chir and the MC was then used)
well that was a bit funny for 1916 innit ?