I know that overstrokes are used to change Coptic alphabet characters to numbers (except for "So," or "6."), and that an overstroke can combine letters for an abbreviation, such as with IC for "Jesus." However, what do the overstrokes of N & M indicate?
In this case, different Coptic dialects have different uses in each dialect. In "Sahidic," for example, they use on top of many letters including M and N. But in the Bohairic dialect, it is only used for the two uses that are correctly mentioned in this question.