Mamiya used a couple of different date code schemes on their products. Most date codes are found on a small gold sticker affixed to the body or lens and consist of two letters. The first letter is the year of production and the second letter is the month. It is located in different spots: many lenses have this sticker on the side although RB/RZ lenses typically have it stuck onto the back of the lens. Some cameras, such as 645 Super, Pro and Pro TL, Mamiya 6/7 and RZ Pro II, have the date code as a prefix to the serial number.
From 1961-1970, the letters A-J were used. Example: AA would be January 1961.
From 1971-1980, the letters A-J were used. Example: AA would be January 1971
From 1981-2000, the letters A-T were used. Example: AA would be January 1981
From 2001-2014, the letters A-N were used. Example: AA would be January 2001
As you can see, the date will also depend on the context of the camera since (mostly) every ten years the code repeated. Knowing a little about when the camera was produced will help decipher the code.
