FrostyTech has posted a review on the Spire 5E34B3 Cu-Baseplate Heatsink

The copper base plate is 4mm thick and attached to the base of the aluminum extrusion by two screws which hold it in place. The area between the copper and aluminum is filled with a silicon-based thermal compound in an effort to improve thermal conductivity. Were the Aluminum and copper to be bonded in the same fashion as the alpha heatsinks there would undoubtedly be less thermal resistance to cope with. Still it seems to be a fairly unique solution to spreading the heat over a larger surface area.

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