When an informal complaint has not been satisfied pursuant to Sec. 1.717, the complainant may file a formal complaint with this Commission in the form specified in Sec. 1.721. Such filing will be deemed to relate back to the filing date of the informal complaint: Provided, That the formal complaint: (a) Is filed within 6 months from the date of the carrier's report, (b) makes reference to the date of the informal complaint, and (c) is based on the same cause of action as the informal complaint. If no formal complaint is filed within the 6-month period, the complainant will be deemed to have abandoned the unsatisfied informal complaint. [51 FR 16040, Apr. 30, 1986]