Hello and welcome, especially as you seem to have an interest in the Thai administrative history same as me. I noticed that map File:Map of thai 1900.png you created, which is nice but has some problems - because you missed to mark some provincial boundaries of provinces which no longer exist now - without checking my lists in detail, at least Lang Suan, Lom Sak, Kabinburi and Sawankhalok are missing, which I had marked in my version File:Thailand monthon 1915.png. andy (talk) 17:01, 25 January 2019 (UTC)Reply

You are right, Kabin Buri did not exist in 1900 yet - it was created in 1907. But the same government act which created Kabin Buri also abolished the province Khlung [1], so you'd have to include that instead. Other provinces which existed until 1932 and thus should be in that map are Sai Buri, Lom Sak, Takua Pa, Lang Suan, Thanyaburi, Phra Pra Daeng and Sukhothai. Only Min Buri was created later, but also abolished in 1932. Note there were also several transfers of districts which further complicate making a fully correct map. I have collected the Royal Gazette references in a XML file [2], though its quite possible that especially in that beginning years of the modern administration not all changes were published there, or that I might have missed one. 16:30, 31 January 2019 (UTC)
For Mukdahan - that province was just created in 1982, so it should not be on any map before that time. In 1917, district Mukdahan was part of Nakhon Phanom province, and thus within Monthon Udon. That 1917 list - taken from the announcement [3] (in XML [4]) sadly does not include the provinces and districts of Bangkok, as those at that time were administered by a different ministry. For Phra Pradaeng (until 1915 named Nakhon Khuen Khan นครเขื่อนขันธ์), that was definitely existing when the census 1919 was done, as can be found in the 1913 yearbook. I am not sure, but maybe that 1921 date you found was when the control over the Bangkok area was transferred to the Ministry of Interior. What source do you use? The Royal Gazette, which is my primary source, seems somewhat incomplete in the early days. A few old statistical yearbooks are available online as well and give some lists, but what would be most helpful would be the full version of this 1913 and other editions of ทำเนียบท้องที่, but those probably only be found in very few libraries in Bangkok, something I wasn't able to look up myself yet. andy (talk) 14:30, 5 February 2019 (UTC)Reply
Checked again for Mukdahan - it was one of the Mueang which were put into Monthon Fai Nuea in 1899 [5]. I also found the source of the 1907 change - that was taken from amphoe.com. Sadly it seems this change was not mentioned in the Royal Gazette, at least I wasn't able to find anything about it yet. Thus I guess what is meant is that the Mueang was reduced in status from being a province under the Monthon, to being a district under Nakhon Phanom. Thus Udon Thani means the Monthon, not the province. andy (talk) 10:39, 6 February 2019 (UTC)Reply