I did not know the technicalaties, only what I was told about. Back on the black and white days, we used a frequency system labeled M. During the change to a color system, there was a need to make sure that the color TVs would still get the b&w signal, thus creating the PAL-M system. I've heard more than once that a lot of effort was put into it so that it would be a good color system for the time. First testings started in mid-60s, and it was only officially used in 1972. I'll try to find out some technical information about it, and post it here.