Help identifying Thai Bullet, probably 1/2 Baht

I'm having trouble with this one. The weight is just over 7g, so I'm thinking it's a 1/2 Baht. The Standard Catalog of World Coins doesn't list any 1/2 Baht bullets in Silver. The folder it came in says:

Siam 1/2 Baht 1851-68 C316

but that didn't lead me anywhere in searches. The circular marks looks like a Chakra from the Bangkok Dynasty, the other mark may be the palace of of Rama III.

Any help would be appreciated.