Google is your friend. http://www.analogalley.com/osccart/product_info.php?currency=GBP&products_id=299
This time, nothing on your Mitsubishi models on hifienhene - except pictures :
From the last pics it is clear output stage is implememented by STKs.
Never saw any Mitsubishi gear in flesh, but some were supposedly quite decent indeed - particularly the modular series predating your units ( around 1989) by about a decade.
I Googled the heck out of both pieces but didn't find anything other than pictures and a few posts on AK about possible wpc. What is "STK"? I'm not familiar with that.