In order to determine that, I have used quite sophisticated mathematical modelling SW,  which is tought in few institutions wordwide :-). With their permission I will publish it here:

1. Download trading data (my source was nasdaq, past 6 months)
2. Ignore all days with BS volume (such as begining of Oct), not related to this excercise
3. Now the trickest part: from each trading day substract volume, you consider to be "an average", I have choosen 3 mil/day, but 2.5 makes it easier :-).
4. Count the excess till you hit 13 mill. shares, which is aproximately the amount of shares Bangchak held.

Outcome: by 30th of November, they were likely done with the job. Another quite obvious fact is, that majority was sold above $11/share and half in a single day (27/11). If not them, then who did sell 6 mill shares on that day?