Except when subdivision (b) of Section 2611 is applicable, in determining the benefit rights of trainees the disability base period shall exclude those quarters during which the trainee was in military service 60 days or more. For all quarters so excluded there shall be substituted an equal number of quarters immediately preceding the trainee’s entry into military service. In the event the base period so determined includes wages in calendar quarters for which the records have been destroyed under proper approval, the claimant may establish the amount of the wages by affidavit in accordance with authorized regulation. The quarter of entry into military service shall be counted as a completed quarter if the director finds that the inclusion thereof would be more equitable to the trainee.