As far as the detail of the decoration, I do not recognize any Thai motifs, which tend to be a little more "flamey" or spikey.  Nor have I read or see a kastane provinenced from Thailand.  There were certainly contacts with Sri Lanka, perhaps more with Burma than Thailand.  One other point to ponder is a royal edict issued in Siam at a time when Japanese blades were becoming common and popular, requiring that the use of such blades would be acceptable only so long as the fittings were Thai.  So you see a fair number of katana blades fitted with handles having a round cross-section and a pommel.  Based on that I would have expected a more Thai-styled handle on the blade, though in more recent times the knuckle-bow saber handle is common. 
 
Coincedentally, here is a sword which was recently sold by Hermann-Historica, attributed to "Borneo/Brunei," but the handle of which to me strongly resembles that of a Burmese dha.  Note the blade ...
		 
		
		
		
			
		
		
		
		
		
		
		
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