Going back to the 747-200, there were both 'M' and 'C' models. The C stands for convertible - Boeing's idea was that these A/C could be used EITHER as pax OR cargo plane. The -200C also has the Nose Door.
The M stands for mixed - both passengers AND cargo on the same plane. The -200M (or -200BM, actually) has no Nose Door, just the Side Cargo Door.
The C model is not offered on the 744, just the Mixed Version.
On the 707, 727, 732, and AFAIK the 73G there also existed C models but no M models. IMHO, they introduced the M designation on the 747 in order to distinguish between the models with the Nose Door and those without it.