During collecting of Thailand revenue stamps during the past 25 years, I have met several times people who did have in their collection Thailand revenues but they had no idea that they were from Thailand.
The older revenues do not have a country name, neither in the Thai language nor in the English language.
Dependent on the interest for this topic, I can make this a frequent regular update of the various issues.
Agriculture revenues. This are the oldest revenue stamps from Siam.
From 1883-1897, a tax was levied on the harvest of rice fields
In the beginning every year there was a different color stamp issued in 6 different values, with the values indicated with Thai words. There are a total of 17 different colors identified
These stamps were printed in France. Used from 1892-1897. Starting 1892, leftover stamps from previous years were overprinted with in Thai numbers 111, 112, 113 and 114, indicated the Thai year Ratanakosin sok (RS). With the year 1889 is RS111
In total, including the overprints there are about 200 different stamps to collect. The common stamps selling now on eBay
for about $5 each and the more rare stamps selling for $150++