This rectangular box, most probably designed to hold betel nut, is of beaten, high-grade silver, decorated with niello work and then gilded (gold plated).
The hinged cover and sides are all decorated with the lai dok phuttan (flower) motif.
The box has obvious age. It was acquired in the UK and most probably came to the UK during colonial times via northern Malaya (Malaysia). There are no significant losses. The lid closes shut tightly and evenly.
References
Naengnoi Punjabhan, Silverware in Thailand, Rerngrom Publishing, 1991.