Pinit Backings for First Day Covers. U.S. Cover #6. 3-5/8 x 6-3/4". .040" thick Acid-Free Board. Indefinite Storage. SKU: KAF40US6PET (0)No Reviews yet Add to Shopping Cart Details Backings for First Day Covers - U.S. Cover #6. 3-5/8 x 6-3/4" .040" thick Acid Free Board. Board is all white. Acid-Free, lignin-free... Details Back DetailsBackings for First Day Covers - U.S. Cover #6. 3-5/8 x 6-3/4".040" thick Acid Free Board. Board is all white. Acid-Free, lignin-free, buffered. Use for Indefinite Storage. Use with polyethylene sleeve SFDCUS6. Reviews 0 Back Ratings & ReviewsNo reviews availableBe the first to Write a Review