Example. Assume that M Corporation, which claims an unused investment credit of $50,000 for the calendar year 1968, files an application under section 6411 of the Code for an adjustment of its tax for 1965, and receives a refund of $50,000 in 1969. In 1971, it is determined that the amount of the unused investment credit for 1968 is $30,000 rather than $50,000. Moreover, it is determined that M Corporation would have owed $40,000 of additional tax for 1965 if it had properly reported certain income which it failed to include in its 1965 return. Assuming that M Corporation filed its 1968 return on March 15, 1969, and that the 3-year period described in section 6501(a) has not been extended, the period prescribed in section 6501(j) for assessing the excessive amount refunded, $20,000 (i.e., $50,000, original amount refunded less $30,000, correct amount of unused investment credit), does not expire until March 15, 1972, and $20,000 may be assessed on or before such date under section 6501(j). Under section 6501(m), M Corporation may be assessed on or before March 15, 1972, an amount not in excess of $30,000 ($50,000, the amount refunded under section 6411, minus $20,000, the amount which may be assessed solely by reason of section 6501 (j)).