The notation goes back to the earliest ASCII Teletypes (ca 1963). There was a CTRL key that toggled the 0x40 bit so that CTRL-M (carriage return) would be 0D instead of 4D, CTRL-G (bell) would be 07 instead of 47, CTRL-L (form feed) would be 0C instead of 4C.
There was no "design" in assigning particular letters to particular functions, it was just chance that, when the dust settled from assigning ASCII codes, the M key was one bit different from carriage return and hence carriage return became CTRL-M.
Here is the best shot I can find of an ASR33 keyboard. As you can see the control character names are printed in small letters on the corresponding alpha keys.
Image may be NSFW.
Clik here to view.
Image by Marcin Wichary, User:AlanM1 (Derived (cropped) from File:ASR-33 2.jpg) [CC BY 2.0], via Wikimedia Commons
The M key does not have a notation on it because there is a dedicated "RETURN" key, so CTRL-M is redundant.
