Mine is black and the serial # is "18-4xxx".
Onepixels serial # looks like "A50xxx".
Off hand from that, I would say black was the first.
All my early Marantz gear like the 10b, 20 and 20b all have, in their serial #, the model then "dash" then number, like 20b-xxxx. They are stamped on a silver plate.
Onepixel, that is a beautiful 18.