Complete set of 16 cards (9 x 3 inches) in original printed envelope. No date--the title lettering is similar to some Kenworthy publications from the 1920s, and the early series number (and more minimal design) suggests it was earlier than the more common 1940s versions. A really appealing design using only number forms with the problem keys tucked into the verso corners--arguably more attractive than later versions that incorporated operations signs and divider. A small closed tear along the fold of the envelope flap, otherwise Near Fine with a pleasant touch of age toning.