Arabic numerals instead of the Thai numerals. These announcements with the numerals I am used to are of course much easier to read for me.
The announcement which I noticed the Arabic numerals was the move of the district office Nong Chok, Phetchaburi in October 1944, when the office was relocated from Nong Chok subdistrict to Cha-Am subdistrict. When in December 1944 the district was renamed to Cha-Am, that announcement contains the Thai numerals already. Checking the other announcements in my list, the switch happened in early December 1944.
The use of Arabic numerals was only adopted two years before, the first announcement I have in my database is the creation of Phra Tabong town municipality (Battambang in Cambodia) in November 1942.
This timing roughly coincides with the Thai spelling reform, which was also in use 1942-44 - under the pressure of the Japanese who occupied the country and had lots of problems with the Thai language. So probably the temporary dropping of the Thai numerals was also related with the Japanese occupation. Nowadays one does not see the Thai numerals in everyday use that much anymore - except the infamous double pricing signs showing the lower prices for Thai nationals in Thai numerals and the much higher price for foreigners in Arabic numerals. Yet official documents like the Royal Gazette will probably the last to switch to Arabic numerals, if ever again.