Changes were also made in Prince Damrong's Thesaphiban system of provincial administration. Five months after his acceptance of the Prince's resignation, the King issued the Announcement of 13 December 1915 concerning the duties and powers of Viceroys. According to this Announcement, the country was to be divided into regions (phak), over which were to be placed Viceroys (Upparat) who were to be appointed by the King. The superintendent commissioners were to remain at their posts but they were to consult with the Viceroys before they undertook to deal with any emergency or any matter not covered by existing acts and regulations, before they submitted annual accounts of their monthons' revenue and expenditure to the Ministry, and before they appointed, transferred, promoted, recommended for decorations, demoted or dismissed officials upwards from the rank of district officer. In that year, the King placed Viceroys over the Phayap Region, which consisted of monthon Phayap and Maharat, and the Western Region (Phak Tawan-tok), which consisted of monthons Nakhon Chaisi and Ratburi. In 1916 and 1922, he placed Viceroys respectively over the Southern Region (Phak Tai) which consisted of monthons Chumphon, Nakhon Sithammarat, and Pattani, and the Isan Region which consisted of monthons Roi-et, Ubonratchathani, and Udonthani.As in a footnote the author gave the title of the announcement mentioned in the text, I was able to find it in the Royal Gazette database - พระบรมราชโองการ ประกาศ ว่าด้วยหน้าที่และอำนาจอุปราช However I could not find anything on those regions introduced later, nor seem there be any list of the Monthon within the two regions established in 1915. But the whole system was rather short-lived anyway, since these regions were abolished in 1925.
As one can see in the map by the yellow parts, these regions with their vice-roys did not cover the whole country - the monthon Phuket and Nakhon Ratchasima were not part of the south or Isan region respectively, and for the whole central and eastern region there did not have regions applied at all.