Bob Dylan and Johnny Cash, Aces & Diamonds
Here's a great find: free MP3s of Bob Dylan and Johnny Cash's unreleased Aces & Diamonds, 1969.
As far as I could determine, this 1969 session features tracks from a CBS Studios session in Nashville, TN that did not see an official release. A Japanese company released these discs from an unknown source. The bonus tracks are taken from The Johnny Cash Show. Cash's vocals have a more commanding presence than Dylan's.