Though he did not find a complete list of these numbers, he could deduce them from statistics at the Department of Land Transport, e.g. the new car statistics sheet lists the provinces in the same ordering as found on the license plates. The strange thing is that these numbers very much look like the TIS:1099 (almost identical with ISO 3166-2) codes, even the first digit for the regions is identical. But the second digit is totally different, for example the upper south using the numbers 80 to 86 have the following translation.
|80||Nakhon Si Thammarat||Chumphon|
|84||Surat Thani||Nakhon Si Thammarat|
The sad thing last - Ian Fuller passed away suddenly in January, so there won't be any updates on this finding by him nor any other of his photo stories. I only discovered his blog one months before his death, so did not have much chance to interact with him. But even from the few things I saw of his net activities his untimely death is a big loss. May he rest in peace.