OPA published notices as to what each stamp was for. It was fairly complicated. Here is an example from a 1943 newspaper.
If you scroll up letlow posted a guide from a newspaper. I just used it to help decipher my book 4. As it was posted 3 years it may be how you missed it. I'll try to repost it here. https://www.cointalk.com/attachments/ration-jpg.517954/