Nice. Those should clean up OK.
Siam changed its name to Thailand in 1932, following a bloodless revolution, so not later than that and probably not earlier than about 1920 from the style.
They’re silver inlaid with “Niello”, which is a low-melting point alloy made from various combinations of copper, silver and lead plus sulphur. It was applied as a paste or a powder to engraved or etched silver and then heated until it melted or was soft enough to flow or be pushed into the pattern marked on the silver and then polished. The sulphur turns it black, creating a contrast to the silver.