“WHY ‘A’ AND ‘B’, AND WHY A1 OR A001…?Cokin A filters have been produced in 3x manufacturing ‘batches’. The original release had numbers A1 etc and the filter no. was printed on most filters. Some were called B instead of A. The Cokin holders have two slot sizes, the slot nearest the camera is for the ‘B’ filters being round in shape. This slot also fits the plate cover. The ‘A’ filters fit any of the other slots and can be ‘twisted’ by turning the holder in the adaptor on the camera. One exception, the A220 ‘Mirage’ or Magic Mirror filter, is actually fitted inside of a hood and not the holder. (This is why I INCLUDE a free hood when posting out). The second ‘Batch’ had numbers like A001, printed on the filters. They are the same filter, but designated with a different numbering system for clarity as the versions grew in numbers. So, just to re-cap; an ‘A1’ filter is EXACTLY the same as an ‘A001’. Round ‘B’ filters fit into the rear slot but are STILL part of the ‘A’ series of filters”