35mm format insert for the negative carrier. Can simply be laid in the negative carrier instead of the glass plate. This allows the negative to be fixed securely in place, and decreases the areas where dust can collect from 4 to 2. If the film doesn't lay flat (like with 6x9 negatives), you have the option of swapping out only the lower glass plate for the negative mask ( a compromise between dust and flat-lying negatives). Aside from that one of the two masks is larger. If you use this mask in the bottom of the negative carrier, or if you buy two sets of negative inserts, you can make full format prints (with black edge around the image).