Much more of these words show up when looking at the Monthon, the administrative circles created in the thesaphiban reforms by Prince Damrong at the beginning of the 20th century. The Monthon Isan covered the southeastern part of the Khorat Plateau, which is now completely called Isan. Monthon Udon around the town Udon Thani covered the northern part of the Khorat Plateau. It also had the Monthon Phayap covering the area northwest as seen from Khorat - now this region is called northern Thailand, historically the Lan Na kingdom. Monthon Burapha were the provinces of western Cambodia, which in 1906 were ceded to French Indochina. Corresponding to this eastern Monthon, the western Monthon was around Prachinburi. I think this is the explanation why a town located east of the center of the kingdom is named "western town".
This naming scheme was even continued at the district level in the northeastern provinces. I discovered it two years ago without realizing that these names are in fact simply directional names. The names Udon (อุดร), Pachin (ปจิม), Thaksin (ทักษิณ) and Uthai (อุไทย) were used - Pachin is only a spelling variant of Prachin, but I don't know the meaning of Uthai, but since two of the districts named Uthai are now the Mueang district maybe it means "central"?