The board shall, pursuant to regulations prescribed by it, refund or credit to a distributor the denominated values, less the discount given on their purchase, of stamps or meter impressions affixed to packages of cigarettes which have prior to distribution become unfit for use, unsalable or have been destroyed, or which after distribution have become unfit for use or unsalable and have been returned

for credit or have been replaced, and the board has proof of that return or destruction.