MCM was a magazine, Miniature Camera MAgazine, in those days miature meant smaller than Quarter plate and foren encompassed 120 and 127 roll film as well as 35,,, 16mm etc is Sub-miniature/
M.C.M. 100 was a developer designed to give extra fine grain with miniature films, it was sold by Johnsons of Hendon, the oldest photographic company in the world, they supplied Fox Talbot, still exist as Johnsons Photopia. M.C.M. 100 was a Meritol based developer, Meritol was unique to Johnsons and a fusion product of Pyrocatechin and Paraphenyldiamine, it was also used in Unitol.
One of the reasons for working at low temperatures is the misguided belief that it helped produce finer grain, there was a German trend to use Rodinal at around 15ºC for this reason. In practice temperature doesn't affect grain size, however it can improve apparent graininess caused by surface artefacts of the emulsion super-coat, these are caused by sudden temperature variations during processing. Most modern films are well hardened so this is now rarely an issue, Acros and the original Tmax400, EFKE and Foma films are/were prone to issues, it's also known as Micro or Incipient reticulation and during printing and particularly scanning the surface artefacts add to the apparent graininess.
Back in Mortesen's day plates and films were poorly hardened and prone to full blown reticulation unless temperature controls were tight, two other factors also affect the swelling of the emulsion apart from increased temperature. on is pH so the more alkali the more swelling. also choice of alkali, Hydroxide softens emulsion far more than Carbonate and Rodinal contains Hydroxide, there's more free Hydroxide now in Rodinal/RO9 since the reformulation after the merger of Agfa with Gevaert in the early 1960's.
M.C.M. 100 uses Borax like ID-11/D76 so there's less emulsion swelling.
Ian.