You might be thinking of the Tomokawa box, which had two exclusive cds both of which have been reissued - 'Songs of Nakahara Chuya' as a stand-alone CD on PSF, and 'Satoru' as an LP on Tiliqua (which is now out of print).
The Mikami box, aside from the ten solo Mikami albums on PSF, had three exclusive cds none of which have been made available in other forms - 'Sings the Piranha Gundan' (Mikami's solo demos for the Piranha Gundan album), 'Dear John' (rerecorded versions of some of his 70s songs), 'Live' (a 1994 show at Manda-La2).