Detlef, I owe you an apology. Here are pictures from an Assam Facebook discussion page showing recently made
heng dang. Wooden hilts, including black shiny ones, and a couple with small disc guards! Also, some with
tunkou it seems!
I don't know how representative these swords are of traditional Tai Ahon weapons, but I think they do seem to confirm the use of plain wooden hilts and perhaps disc guards on these swords. It would be nice to find some museum specimens with established provenance. However, your Assam designation is in play. Who knew? Amazing what is out there in the digital jungle.

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