A used school bus not previously on the reimbursement system shall be placed on the reimbursement system as if it had been issued a school bus operation permit on the first of September in the year of manufacture. A used school bus previously on the reimbursement system shall be placed on the system using the original initial eligible month and assuming no break in eligible months. The superintendent shall calculate reimbursement for used school buses using the assumption that all possible prior system payments were paid along with any assumed interest earnings.