As part of the Southeast Asia Fertility Program the census data of Thailand were analyzed, and several files placed online. One of it contains the numerical codes for the districts, using a two-digit system with the first two digits designating the province, and the next two the district. A leading "0" for the province is omitted however. Therefore the codes look similar to the DOPA codes, however both provinces and districts start at 1 without any holes. The system of the numbering for both province and districts is also easy - simply the Thai alphabetically sorting. Only the capital district (Amphoe Mueang) always gets the number 01. Also notable - the romanization does not follow RTGS (was that standard around in 1990 already?), but uses its own inconsistent system. Also a noteworthy special case - the special administrative area of Pattaya has the number 911, even though it is not a district but a special kind of municipality.
As an example, here are those codes for Chonburi including that special code for Pattaya. The "KA" at the beginning is the abbreviation for "King Amphoe" (กิ่งอำเภอ, minor district).
901 'CHON BURI ' 902 'KA.KO SICHANG ' 903 'BANG LAMUNG ' 904 'KA. BO THONG ' 905 'BAN BUNG ' 906 'PHANAT NIKHOM ' 907 'PHAN THONG ' 908 'SI RACHA ' 909 'SATTAHIP ' 910 'KA. NONG YAI ' 911 'PATTAYA 'The site also includes the codebooks for the censuses 1970 and 1980, but I can only find the province codes for these. However these also seem to be simply alphabetically sorted, so I suppose the same applied to the district codes. But since between each census it had new and merged provinces and districts, the codes from each census is not compatible with the next and one has to manually match the codes. Quite easy to see the advantage of a temporarily stable coding system here...
The codebook PDFs also mention codes used for subdistricts, municipalities and sanitary districts, but sadly I have examples yet. I will therefore postpone that subtopic to a later posting, let alone to keep this posting at a readable length.
I can only hope that for the 2010 census NSO will change to use the DOPA/TIS codes. The fact that in 2000 they used a system based on the FIPS 10-4 codes shows that they have noticed the need, but why they chose that US standard instead of the ISO and Thai standard (TIS 1099:2548) remains a mystery.