So I checked with the website which has both the postal codes as well as the most complete list of the Muban (as far as know), but to my surprise this also lacks Mu 9 and 10 [Source]. This might still be possible if these two Muban are part of a town or city, and thus have been abolished altogether. Next step was to check the Royal Gazette - end of the 1990s the boundaries of every Tambon were defined in announcements by the Ministry of Interior, and these contain both the description of the boundary as well as the names of the Muban. The 1997 announcement for Tha Ruea district lists 10 Muban for Nong Khanak, thus ends with the two names I was looking for.
However, this isn't the end of the story, because the TAO covers the whole Tambon, so there is no municipality which could have led to the abolishment of these two Muban. The TAO website even includes a map, which clearly shows no trace of these two Muban. Taking a look at the names of these two Muban is also interesting: Mu 9 was named Khok Khram (บ้านโคกคราม), and the postal code list names Mu 11 as Khok Khram with the alternative name of Bo Rae (บ้านบ่อแร่), whereas the TAO website names Mu 11 just Bo Rae. Similar for Mu 9, in 1997 named Nong Muang (บ้านหนองม่วง), the same name is listed for Mu 12 in the postal code list, and the TAO names it Nong Buang (บ้านหนองบ่วง).
It thus looks like as if these two Muban were renamed and received new numbers, something I never seen before - normally Muban don't change their number if they change name, the only occasion when Muban changed numbers were when a Tambon was split and the Muban in the remaining area are changed to be consecutive again.Thus, if anyone can explain me this special case, I'd appreciate it.
In my XML I have encoded this oddity by marking Mu 9 and 10 as obsolete, this way it will not look like these numbers are really missing, but will still calculate the number of Muban as 10.
<entity type="Muban" geocode="14020909" name="บ้านโคกคราม" english="Ban Khok Khram" obsolete="1" /> <entity type="Muban" geocode="14020910" name="บ้านหนองม่วง" english="Ban Nong Muang" obsolete="1" /> <entity type="Muban" geocode="14020911" name="บ้านบ่อแร่" english="Ban Bo Rae" /> <entity type="Muban" geocode="14020912" name="บ้านหนองบ่วง" english="Ban Nong Buang" />