Wow that's very rare. I have a small collection of early Sony Discmen including the D20, D-50, D-Z555, D-88...etc but I've never come accross this one before.
From the styling it looks to be a contemporary of the D-50 ( D-5 in the USA) which would be around 1985-6 by the time it was comercially available worldwide although models of this vintage were around since the early '80s and were practically prototypes.
As such build quality is generally very high as mass production hadn't begun in earnest and you often find many of the parts had to be bespoke. Sound-wise I really like the signature of the first generation Sony PCDPs. To my ear it's often preferable to the full sized Sony units of this period which were not as musical as the early Philips based machines. I'd expect yours to sound like the D-50 which has a rich warm slightly rolled off sound with really impressive bass compared to a modern portable CD player ( if they are still available) or MP3 player.
Machines like this don't offer the levels of detail which the best modern DACs can extract but they are still often musically very satisfying and the superior build quality generally imbues them with a sense of solidity lacking in all but the very best high end CD transports available today.
Products like this were completely focused on squeezing every last ounce of music from a CD, coming almost hand built straight out of the R&D department of a Sony at the peak of it's powers, as it's statement product for the whole company. From an engineering point of view this is pretty impressive stuff and it will appreciate in value to serious collectors of this kind of stuff which as you no doubt know from walkmans there are many.
Presumably you're familiar with sites likeTVK °1: SonyWalkman Central - Sony Walkman ReferenceVintage Electronics Have Soul - The Pocket Calculator Show Website
where you can read more about this sort of stuff. None of them seem to have an example of your discman so I'm sure there will be considerable interest there. They do have some nice pics of your Biotracer though.