Mainstream was an extension of Bob Shad's earlier Time label.
Cobblestone was the brainchild of Joe Fields, then marketing director for Buddah. Joe had broken into the record biz as salesman for Bob Weinstock at Prestige. I met him in this capacity in the mid '60s. Anyway, Joe convinced Buddah to start a "Prestige-like" sub and he brought Don Schlitten in to produce. When Joe left Buddah he negotiated some sort of deal to take most of the Cobblestone masters with him. He then started Muse with Schlitten as a minority partner. After a couple of years Joe and Don had a messy "divorce" complete with a "years long" series of lawsuits over the ownership of certain Muse and Onyx masters. This was not settled until the mid '80s. First evidence of the settlement was the Xanadu release of an unissued Muse session by Terry Gibbs and Joe started releasing some of the old Onyx stuff.