My earliest was the C712. I don't think t was the first though. Take a look at these two links and you may find the answer.
http://www.berners.ch/McIntosh/en/Matrix.htm
or
http://www.roger-russell.com/aboutmc.htm
http://www.berners.ch/McIntosh/en/Matrix.htm
or
http://www.roger-russell.com/aboutmc.htm