Piotr I should have
looked at the map before hitting the keys. I had imagined that
Myanmar represented an unrelated linguistic barrier between Assam and
Thailand but clearly that is not the case. I know little of
the history of the area but I suppose that the modern frontiers bear
no relation to any previous divisions, if any, except perhaps where
rivers form a natural boundary.
I am now considerably better
educated than I was two days ago. Thank you for the information which
I think answers my original enquiry perfectly. I will therefore
un-subscribe and not clutter the site any further.