I knew that back at the beginning of the sanitary districts in Thailand, there were two kinds of these districts created. At first the urban sanitary districts (สุขาภิบาลเมือง, Sukhaphiban Mueang) in the urban areas, and the rural sanitary districts (สุขาภิบาลตำบล, Sukhaphiban Tambon), both officially codified in law by the 1914 Local Administration Act. As an example, the first rural sanitary district was Tha Chalom, the precursor of the modern day municipality Samut Sakhon, whereas the first urban sanitary district was located within Bangkok. In 1935 all 35 sanitary districts were upgraded to municipalities (Gazette), and for the new sanitary districts created since 1950 I never knew about any distinction between those entities.
But now I found in one section of the book The Management of Secondary Cities in Southeast Asia on the Urban Areas in Nakhon Ratchasima province a list of the various kinds of municipal areas, which includes 17 town (urban) and 20 rural sanitary districts. This book is from 1996, so just few years before the sanitary districts again were all upgraded to full municipalities. Thus these two levels must have been in existence for the second generation of sanitary districts as well, probably this act wasn't amended ever since on this issue. However, all the announcements on the creation of sanitary districts only use the word Sukhaphiban (สุขาภิบาล) without specifying the level, for example the 1956 creation of sanitary district Khanom (Gazette). Thus the thing I need to find now is a list of the 980 sanitary districts with the information of whether they are urban or rural sanitary districts.