The latest version of the Thai Industry Standard 1099 covering the geocodes includes codes for districts and subdistricts as well as the municipalities, while the first edition only covered the provinces. Those for districts and subdistricts are identical with the list published by the Department of Provincial Administration. However the municipal codes from TIS 1099:2548 are not in that list.
I had found these geocodes for the municipalities before on a list by the Office of Narcotics Control Board, not the place one would search for it. When I found them in the tables of TIS 1099:2548, it was good to see them confirmed by something official. However as I already noted this system of codes cannot be used anymore, as with the many TAO upgrades recently at least Nakhon Ratchasima already has more than 100 municipalities and districts, thus not enough codes to cover them all.
The main difference between the TIS 1099 codes and those of ONCB is the fact that the ONCB was much more complete, as it included all of the municipalities which existed around the year 2005. TIS 1099:2548 instead only has very few municipalities for each province, often just a single one, and the maximum being 7 codes for Chonburi - 2099 down to 2093. As this version of TIS 1099 was published in 2005 (B.E. 2548) it is strange that most of the municipalities which existed then were not included. So I systematically checked all the codes in that list trying to find the rules which municipalities were included.
In fact the rule quickly showed that it is only the municipalities before the upgrade of the sanitary districts in 1999 which are included. Also none of the municipalities created by upgrading TAO in after 1999 is in that tables - and even status changes which happened between 1999 and 2005 are missing. For example in Chonburi the town municipalities Ban Bueng and Saen Suk are listed as subdistrict municipalities, though the were upgraded in 2001. Thus it seems that despite the name TIS 1099:2548 the real data in that standard dates from no later than 2542, in other words was at least 6 years old. Luckily, at least the district and subdistrict table was still up to date, but only because no new such entity was created since 1998.