Colin:
This simple style of wrapping with broad strips of rattan/bamboo is seen on some Naga items that appear to be more working tools than weapons (see
here for a few examples). The better quality
dao, which this one seems to be, usually have smaller strips of rattan/bamboo that are woven in a more intricate fashion than the wrap on this one.
Your
dao also seems bigger than the usual Naga
dao, which you note. I have not seen one exactly like it before. Your estimate of Assam or neighboring tribal areas of NE India does seem a likely origin.
Gavin has some experience with
dao from these areas and may have seen a similar example.
Thanks for posting this unusual sword.
Ian.