If sufficient records do not exist to accurately determine under Sec. 7.465-2 the amount which a taxpayer is at risk on the effective date, the amount at risk shall be the taxpayer's basis in the activity reduced (but not below zero) by the taxpayer's share of amounts described in section 465(b) (3) or (4) with respect to the activity on the day before the effective date. [T.D. 7504, 42 FR 42198, Aug. 22, 1977]