From a Graflex Service Bulletin issued July, 1966

Each Graflex product is stamped with a three-digit code showing date of manufacture. The first letter is the month, the second numeral is the year, and the third letter is the revision. The revision letter is changed when a new model is introduced or when a major modification is made to the equipment.

A product with the letters A6C would be one manufactured in January, 1966 to the 3rd modification.