Information about the Limited Prologue 21 has been difficult to find. This model is extremely uncommon, I have only seen 3. Of those, I have serial numbers from two, both appearing to be assumedly produced July 1997 and are less than 500 units apart. The only differences I have found between the Limited Prologue and the Super Prologue is in branding, though the one Limited I own has had the HDD previously swapped by SMN to an upgraded IDE drive and may not contain original data.
SEGA Prologue 21 