What is wrong with my iPhone's model number?
An N in place of M simply means it's a replacement unit from Apple. As per this wiki article:

The first letter in the model number represents the source of the device. Currently, there are four known possible choices:

F: refurbished unit.

M: retail unit.

N: replacement unit.

P: personalized (engraved) unit.



You can view the model number for an iOS device by going to Settings → General → About → Model on your iPhone/iPad.
The default string displays the iOS device part number. Tapping on the model cell shows the model identifier string.
To identify an iPhone by its model number, refer to the Apple support article, Identify your iPhone model