In American English, No is a "standard abbreviation" for North.
For instance, see American Heritage Dictionary, which gives the two-letter abbreviation with a period (No. or no.)
In Arkansas the state capital (Little Rock) is contiguous to the city to the north called North Little Rock, and this latter is often abbreviated No Little Rock on street signs and addresses.
For example, see Snagajob and remember that in this context No Little Rock jobs doesn't mean there are no jobs in Little Rock; it's referring to jobs in North Little Rock.
And Google understands No St Paul High School to refer to the high school in North Saint Paul, Minnesota.
The phone number 308-526-xxxx is located in NO PLATTE, Nebraska. (NPNR) That's North Platte, Nebraska.
I think the use of No (or No.) in such contexts is because a two character abbreviation is a bit longer and easier to read or harder to confuse, especially in hand writing, but I'm not sure.