AzurePark:
Thank you for bringing this interesting sword to our attention. The blade and hilt of the sword suggest that it is from N. Thailand/Laos. The scabbard is rather nondescript, but also consistent with that general area. It is a cut above the usual village quality Montagnard swords which have, as Ariel noted, a hilt to blade length ratio of about 1:1 and a blade terminus that widens but is less rounded than this example.
By way of example of the more usual forms of Montagnard daab, I have attached two swords that were exhibited at the Macao Museum of Art "History of Steel in East Asia." The images of these swords are taken from the exhibition's
web site where you will find further examples of Burmese and Thai swords.
The third picture below with the black background is a sword of mine that shows a N. Thai/Lao daab, probably from the 19th C or perhaps earlier. The hilt has some similarities to the one you show.
As to the age of your sword, it is hard to judge without having it in hand. The style, however, is quite old and the bronze/brass hilt suggests to me it is at least 19th C. This is not your every day Montagnard work sword, but one of some distinction and probably an heirloom that was highly valued by its indigenous owner. Such a sword may well have been owned by an influential Montagnard chief. The gift of such a sword would have reflected the esteem in which the recipient, Sgt Thornhill, was held by the chief.
Thank you again for sharing this interesting sword and its story.
Ian.