I don't know what they're called, but clamp makes sense to me.
I've been looking for a clamp that attatches to the heron bracket as well. It seems they came on the lamps, as already mentioned, but it also seems like they can be separated from the lamps.
I figure I will end up rigging a bracket that bolts through the heron bracket, or wraps around it as the original clamp does since they are hard to find.