I agree. The initial assumption that a square/concave tip mean Kachin has not held up. I have seen a couple with the double circle mark, which I think represants a flower. I have seen a few more with a single circle. The meaning of the marks I have no idea about, though I am assuming that they are maker's marks. They seem to occur on dha from northern Thailand (e.g., Chiang Mai), however.
Here are two circles combined with a turtle; note the stylized foliage extending from the right-hand one
Here is one that was filled with resin
Another single divided like a pie, made with a single punch it seems
