Many, many years ago I had a friend and classmate who owned a 150 DHC. I can remember him saying it was a 150 M and explaining the difference between an M and an MC. I have tried from time to time to confirm his nomenclature without success and to find definitions. Could someone shead some light, please.

As far as I know the only Jaguar car with an 'M' was the Mark VIIM saloon of 1954. This was a follow-on from the original Mk VII of 1952. The Mk VIIM had auto transmission as an option for the first time.

https://en.wikipedia.org/wiki/Jaguar_Mark_VII

There were XK150 S models from 1958 with a more powerful engine due to a new cylinder head being fitted, and three carburettors

Thank you, I was unable to find any support for the use of an M or MC designation with an XK 150. But I wanted to check with the experts on this form.