If you post a scan, I can help you to identify revenue or Cinderella stamps from Siam/ Thailand and give you a best value estimate
Below revenue stamps from Kelantan in Malaysia issued during the Thai occupation from 1943-1945
In 1943 the Japanese ceded the administrative control of the Northern Malaya states including Kelantan to the Thai. This as a reward for their "co-operation"
The stamps shown are revenue stamps, top line on the stamp reading Kelantan in Thai language, the bottom line reading Hasil = revenue in the local Jawi language. The value is in the the bottom line.
These stamps are very rare. There is a similar range for postage stamps except the bottom line reads Postah and the coat of arms is printed not in red but in a purple color. Also for the postage stamps the highest value is only 10 cents while the highest value for the revenues is $50, 500 times more!