79pmooney, Old Man Mountain might still have the piece that mounted to the canti bosses. The two upper rack stays mount to the side of the rack and then to the adapter. It is/was used with a Santana tandem. The adapter piece was U shaped.
I adapted my OMM Santana rack to mount without the adapter onto my touring bike.
Brad
PS I found this to help explain:
http://santanatandem.com/Library/RearRackInst.pdf