Below is a list of unreleased prototype MD units and info on where they were mentioned:

Stereo Review Stereo Buyers Guide 1993 pg.6

There were 1-2 other MD prototype units from the early days.

Kenwood DM9090 Firewire prototype deck showed at Comdex 1998.

Hi-MD prototype ( av.watch )

Bose DCC / MD head unit Includes a (4?) MD changer!