|กุดน้ำใส||Kut Nam Sai||35070200||35069800|
The additional codes starting with 3506 would be geocodes of these subdistricts while they still belonged to Maha Chana Chai (which has the code 3506) - the minor district Kho Wang was established in 1975 (Gazette).
That would be a good explanation for these codes, but there is one problem - in several cases these codes are wrongly assigned. Sometimes not all the original subdistricts of a new minor district have such an obsolete geocodes, but even more striking is the other way round. If reading the announcement of the creation of Kho Wang, one notices that only three subdistricts were split off; the fourth subdistrict named Kho Wang was in fact created in 1979 (Gazette). Thus, in fact this subdistrict was never part of Maha Chana Chai, and therefore should never have had a geocode starting with 3506.
Another example - Kap Choeng district, Surin, was created in 1975 consisting of the subdistricts Kap Choeng and Bakdai taken from the district Prasat (Gazette). However, the subdistricts Khu Tan and Dan, which were reassigned to this district from Sangkha district in 1977 (Gazette), both have two such obsolete geocodes, one for Sangkha district and one for Prasat district, even they never were part of Prasat. However the subdistrict Khok Klang created in 1976 (Gazette) does not have any such an obsolete geocode at all.
So I can only guess that these codes were an attempt to extend the geocode system into the 1970s, but while assigning the codes quite often mistakes like the above were made. And not only when there were these codes there were mistakes, there are also districts which were created in the 1970s but don't have such obsolete codes. Too bad I can only make my guesses from the numbers and the historical facts, I would love to know more on what is really behind these (and other) strange geocodes.