Gavin:
You are absolutely right. The Kachin have used Shan style dha for a long time. That the British had Burmese/Kachin troops wearing Shan style dha is well documented in the photographs provided by Spiral, and there are many 19th C pictures that confirm the civilian use of Shan dha by the Kachin.
Spiral's photographs of British uniformed troops with Shan type dha indicate that the British military in charge supported the use of such dha, and may have provided the dha or at least given the men an allowance to purchase a dha locally.
The picture of Kachin Rangers shows men wearing several Shan dha, the one on the left having a "parade style" silver-hlted version, and the others wearing the working man's dha with a wooden hilt covered by thin rattan strips. From the appearance of the hilt and scabbards on some of these, they appear to be fairly new.
With respect to the items in the catalog of William Hunt & Sons, I have done some online research on this company. The company did have a branch in Kenya and there is reference to a catalog for sales in Nairobi, but I have not found a copy of it. There is an online version of their 1941 catalog that shows the various die marks the company has used over the years--you can view the file
here. I have never seen any of these marks on items coming from SE Asia or Southern Asia. Unfortunately, I was not able to copy the page with trademarks because of copy-protection on the PDF file. However, you can see by scrolling down a couple of pages into the file.
As far as the question regarding documentation of the US or British military supplying dha to the Kachin Rangers, I don't recall where I read that. Possibly in Carter Rila's article, or it may have been in one of my books about the Chindits. I last researched the topic when putting together the article on
Contemporary Thai Swords on the old forum. In any case, I will try to find the reference for you.
Lastly, the 1940s-era machetes that you show are, as you say, commonly referred to as "Chindit machetes." I have only heard of them used in relation to the Chindit forces under Wingate. I've attached a picture of one of mine, not sure if this is a MkII or a MkIII with a MkII hilt.
Ian.