In fact, the Onkyo 105fx used at least two different Sony MD transports with different DSP. The old one uses KMK 400AAA and is unique.
CXD2686-223GG - Onkyo MD-133, 105FX < 2007, many other bookshelf Onkyo with Hi-MD;
CXD2687-001GG - Sony MZ-RH1, Onkyo 105FX > 2007;
CXD2683-223GG - Bookshelf Sony CMT-AH10;
And KMK 400AAA with the Sony DSP CXD2686-223GG does have DOUT pin, top right board edge behind the memory chip, near pin 24.
So, it could be used with @kgallen optical board project I believe.