Yes there is no mention anywhere of any details. typical wink
Basically the optical O1-SAT at the bottom left corner of the unit has been replaced by a USB input most likely at the end of 2007 (according to the date on the datasheets for G68 at the time).

The connection supports 16/48 (page 12 of current G68 Surround Controller PDF) so my guess now is that it is a cheap and quick adaptative implementation...

it would have been great to have a proper 24/96 implementation, may be it was a rushed job to get one in quick.