i was having a look over at hou de and came across this :
Q: What does 7542 or 8582 mean?
Started in 70’s, Yunnan pu-erh producers started to use this special coding system to trace and manage the production of huge varieties of pu-erhs. The following figure explains the basic formula of this system:
With the code 7542, "75" means that this recipe started being used in 1975. However, 7542 green cakes have been produced every year since 1975! "4" refers to the special "Blend" that was used to make this cake: raw leaves after harvest are usually sorted into 10 different grades. Then, the tea producer, following the "recipe", blend several grades of leaves together. Of course, sometime a single grade is used for a "blend". They may use one blend for the surface of the cake, and the other blend for the body. For example, 7542 green cake is usually made by 3~4 grade "blend" for the surface, and 5~6 grade "blend" for the body.
The fourth digit, "2", refers to the producing tea factory. As explained in the above figure, "2" refers to Meng Hai Tea Factory. Mostly you will see 1 ~ 4 as the last digit, but when a new tea factory being strated up, he will want to pick his own number if he is important enough.
So, "8582" means that they used a recipe that started in 1985, Level 8 blend, and was produced in Meng Hai Tea Factory. Usually you can tell from the code if the cake/brick is cooked or uncooked: for example, 7542 and 8582 are both uncooked(green) cakes, and 7572, 8592 and 7663 are all cooked pu-erhs.
The last three digits, XYZ, are usually not shown and hard to determine! But, definitely, even with the same "7542" different batches may exihibit different characetristics! "7542" during 70s had four different batches: 205, 206, 503, 504. Sometimes one batch is more compressed than other ones, especially if they were all compressed by stone molds instead of machines. The weather conditions also affects the characteristics of different batches: one batch may be sun-dried in a very sunny day, and the other batch may have to be assisted by burning pine woods to dry it. The later batch will naturally have more "smoky" feeling to it than the sun-dried batch.