We're at the edge of my expertise here, but I believe the 6A and 7A designations were sizes of slip-on devices that could be filters or diopters. I (probably still have) a closeup lens for my Brownie Target that I think had a 6A designation, but a quick search shows things on ePrey in original Kodak boxes stating 'Closeup Attachment 6A' and another, 'Cloud Filter 6A', so the 6A is not a fully qualified identifier. Dunno, maybe somebody pried some stuff apart to make one of the nA push-on close-ups into a series filter -- weird! (Which is also annoying because push-on stuff seems to be increasingly rare, hate to think they are being scrapped!)