Katana,
I'd have to agree with Mark on this one Shan or Northern Thailand the spine filed in that shape is something I've only seen on swords from those areas, most of the swords in Central Thailand either have a flat or rounded spine.
Many have the cross-hatch and inserts and while there may be some significance to three of each (most I've seen have three) and the Thais believe 3 is a lucky number (at least thats what the gamblers keep telling me) actually all odd numbers are important, can't tell you why but if you look at the nobles umbrella they are all in odd numbers 3, 5, 7, 9 depending on rank. The Erawan (elephant god) is depicted with three heads, Nagas again start with three and go up by odd numbers depending on rank.
The one Mark shows in all in twos (maybe that was the original owners lucky number) but I still doubt that their placement on the weapon is more than decoration.
I've seen fullers running 2/3 to 3/4 length on central Thai swords but they are usually very thin and not deep, yours is more pronounced and with the wonderful scroll work again leads me to believe North.
The handle is very common but does not look Northern Thai. I've seen these in great numbers in Central Thailand (i'll have another look after I finish this) but I'm wondering if it got rehilted, either way the braided grass rings would have gone all the way down to the pommel and if the tang does in fact extend that far into the hilt it would longer than most I've seen dismantled.
Either way it is definitely a true ethnographic weapon and a nice start for someone on the road to the Dha(K) side
Dan