Okay, just going to reel this in a bit and focus.
I could be wrong, but i don't think that this is an American board depicting the great god the atom Enstein, nor is it from a Malay tajong design or the Maori of New Zealand.
I am fairly sure it is indeed Dayak. I am also fairly sure that it should, to some extent at least, be possible to determine what tribe it is based on the actual design of the carving. I have seen some of our Dayak collectors tell tribal origin of mandaus based on the carving of the hilts so it seems to follow that design styles are specific to certain groups. I would also be very interested in understanding what the purpose of the basket this was attached to might have been. The one Gustav shows might provide some clues to this as it seems to be in a museum. Was there any descriptive labeling?