The letter "M" is a tough one for web searches... Here are some links:
http://cycling74.com/products/m/
http://cycling74.com/support/faq_m/
http://tamw.atari-users.net/m.htm
http://createdigitalmusic.com/2007/06/22/m-interactive-composer-retro-software-now-intel-mac-native-core-midi-ready/
I also have a few cool articles and reviews in old issues of Electronic Musician and Computer Music Journal, maybe I will scan sometime.
Update on the old Mac front: I am still missing bits for my Plus here somewhere, but I bought an SE off of a guy for cheap, complete with accessories. It even came with a Symbolics mousepad, which caused me to get just a little too excited! This week I will try to set up a hard drive and test the floppies. Got to be slow and careful since these are key diskettes. I am all too well aware how lame it is to ruin a key disk. Also I scored a small stash of Emagic dongles. The oldest is for Notator and has ADB and also connectors for Mac Plus keyboard, then there's an ADB dongle which still has the old Emagic logo, and one with a newer logo and is USB only. The USB dongle is a pre-XSkey version which means that it must be for Logic Audio 4.5-4.8.