Hi Iain, I know, it is perfectly usual for such horn to be used throughout the history of Siam and other countries throughout the region on knives large and small, China to, Sambar stag horn to as the type of deer were in south east Asia, and it is sambar stag on these hilts for sure.
Here are just a few pics to back up the idea of the royal barge weapons, back slung swords etc, I see no reason why these could not be pairs, or singles, steersman were known to carry paired swords on the royal barges, but there is little information about them in English or Thai available.
Or perhaps they are from another region further south where other klong barges were used with fighting men aboard to guard royals and high ranked officials etc.
Royal vanguard marines also carried swords and some in pairs, could be these are from one of the many barges filled with up front men who all carried swords and other weapons as royal guard and police waterborne marine vanguard clearing men, I will likely never know the whole storey behind them.
Siam was a place with many klongs - canals and waterways, still is today, many waterborne armies used them as was the same in Cambodia in the 19thc into the early 20th which is the date I would suggest for my pair of swords, and also the large knives, which actually do feel like back hand fighting blades in the hand, or badek -badik pistol grip types.
I could add so much more but for now this will do to show some similar types in use in earlier times in old early 20thc pics, and some from today.
