The monthon (circles, มณฑล) were an administrative entity above the provinces, created in the administrative reforms of Prince Damrong in the 1890s and 1900s. Ever since I first read about them while collecting information for a province article on Wikipedia I wanted to find out more about these entities, and the difficulty to find information on English only made it more interesting. So I finally got myself the book The Provincial Administration of Siam 1892-1915 by Tej Bunnag from a library and got many new insights reading it - just too bad its out of print, and none of the antiquarians ever had one yet. However as the title of the book already suggests, that book ends with the resignation of Prince Damrong from his post as interior minister in 1915, and only mentions shortly that some of the monthon were abolished in 1925, and then finally after the revolution of 1932 all were abolished. So it was a minor surprise when I read the Royal Gazette entry on the abolishment of several provinces in 1932 and found out that at the same time also the monthon Pattani, Chanthaburi, Nakhon Chai Si and Nakhon Sawan were abolished and the provinces under these monthon assigned to the remaining monthon. According to a poster in the 2bangkok forum the final abolishment was a retaliation to the monthon administrators supporting the Boworndej counter revolution attempt.
The map shows the monthon as of 1915. I created the map in Tej Bunnags book to illustrate the Wikipedia article on the monthon. But actually the provinces shown in that map are those of 1932 after the abolishments mentioned above, so as soon as I find the districts within these provinces I will create a new version of that map.
Too bad my Thai is still so bad, browsing through the Royal Gazette would give many new insights or small facts almost impossible to find in English, or garbled in sloppy translations.