25 C.F.R. § 135.5
Penalty
To all assessments not paid on the due date, there shall be added a penalty of one-half of one percent per month or fraction thereof, from the due date so long as the delinquency continues.
To all assessments not paid on the due date, there shall be added a penalty of one-half of one percent per month or fraction thereof, from the due date so long as the delinquency continues.