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
The 1991 c37 is vintage, maybe not the oldest, and is a step up in sound from their earlier models. I love mine, but I'm missing the HR37 remote, and they are hard to find, so if you buy a vintage Mac make sure it includes the remote.