I am sorry to say, this is almost certainly a fake. I have no idea what "Siamese history" is being referred to in the description, but having done more than a little research on Siamese history, and in particular martial history, I have never seen a reference to such a sword. The illustration is of a scene from the life of Gautama, the attack of Mara and his demonic hordes attempting to prevent Gautama attaining enlightenment. These are invariably very fanciful in depicting the demonic side, including weapons used, so I don't place much credence in the painting as reflecting anything historical.
To my eye there are a number of big red flags in the photos and description, including the close-up of the break, the poor execution of the fitting decoration, an evident (to me at least) lack of any third-dimension geometry (the blade looks flat, in other words) and the very rough execution of the blade. They hardly shout "spectacular," or "authentic" to me - I am immediately suspicious of such hyped language, anyway. Though perhaps in other contexts a blade roughly cut from a flat piece of steel would be a genuine article of historical manufacture, that was not something that was done in SEA until late in the last century.
If they truly guarantee the authenticity of everything they sell, and provide a certificate to that effect, you should have a legitimate claim to a refund, as the blade on in-hand inspection clearly is not an historical item and they have not provided the certificate. I am curious to know the identity of the "expert in SEA arms & armor," but I expect that if you challenge the seller to name him/her all you will get is evasion.
By the way, feel free to cite me as a recognized expert who says it is junk.
