Dear Nick,
The codes vary a bit depending on the language. Also, we sometimes add codes to further differentiate word classes. There is a further discussion of this in the article cited at the top of this page
http://childes.talkbank.org/morgrams/
Each part of speech is listed in a separate file in the mor lexicon for each language and the best way to understand the actual practice in detail is to study the forms in the lexicon. Also, in the English grammar for example, the first 40 lines of
the cr.cut file list all the parts of speech codes being used.
—Brian MacWhinney