Except as provided in subdivision (b) of Section 2611, in determining the benefit rights of any industrially disabled person the disability base period shall exclude those quarters during which such person was industrially disabled for 60 days or more. For all quarters so excluded there shall be substituted an equal number of quarters immediately preceding the commencement of his or her industrial disability. In the event the base period so determined includes wages in calendar quarters for which the records have been destroyed under proper approval, a claimant may establish the amount of wages by affidavit in accordance with authorized regulations. The quarter of commencement of an industrial disability shall be counted as a completed quarter if the director finds that the inclusion thereof would be more equitable to the industrially disabled person.