Okay, three years late on this, but hey, I just joined today.
The mark on the upper right is indeed Thai. It reads เงินดี (ngern dee) meaning "good silver" which is used to mark high content silver pieces. I'm not sure if that starts at 90% or at .925, but probably 90% plus.
Likely from a Chinese silver maker in Thailand, very common. That's really all I can tell you about this very nice looking piece.
Btw, if the writing is clear and sharp, I can read Thai and Lao, but not Cambodian (Khmer), even though it does share similarities.