Section 28. Each credit union shall preserve all of its records of original and final entry, including cancelled checks and withdrawal and deposit slips, for a period of at least six years from the date of making the same or from the date of the last entry thereof. Such records shall be preserved in their original form for two years and thereafter may be preserved on microfilm or microfiche. All original records, age three years or older, may be stored in off property locations; provided, however, that withdrawal slips may be destroyed after a period of two years, if such withdrawal slips are properly microphotographed and the microphotographs shall be preserved for the period provided in this section.