Sony bought Mainstream because it released the first Big Brother and the Holding Company album, and Columbia wanted to add it to their Janis properties. They pretty much ignored the catalog except for repackaging some Sarah Vaughan & Carmen McRae albums (both were at their peak during those years). It appears that some titles have been released through the Japanese P-Vine label. I love Charles McPherson's Today's Man, with great Ernie Wilkins charts.
IIRC, Mainstream had a "Time Series" for rereleases of Time material.