Eric Muller notified me on one peculiarity of the FIPS codes for the Thai provinces - unlike almost all other countries the codes are assigned in the alphabetical order of the romanized name.
I haven't been able to find any sources which give a rationale for the coding scheme used, from the original codes used in FIPS 10-3 it seems a geographical ordering was used. The codes start with Mae Hong Son (TH01) in the northwestern corner, then continue through the northern provinces, then the northeast, central, east and finally southern provinces, ending with Yala (TH-70) as the southernmost province. But then there are the codes 71 and 72 for Ubon Ratchathani and Yasothon out of the order, and Narathiwat being placed in between the northeastern and central provinces as TH31.
Also interesting is the fact that the province Mukdahan, which was created in 1982, was not included in the original codes, only with the change from revision 10-3 to 10-4 in 1997, together with the three other provinces created in 1992. As FIPS 10-3 dates from 1984, this change might not have propagated into the sources used for creation of these codes by then, and none of the minor changes of the 10-3 codes catched it.
As it was custom in the FIPS coding scheme, with the split of a entity both the new entity as well as the modified one both get new codes assigned. There the change to FIPS 10-4 had the following code changes
|TH79||Nong Bua Lamphu|
But anyway the whole thing is like beating a dead horse - the FIPS codes are no longer maintained and the move to the ISO 3166 codes is recommended.