How do I find my mobile country code when the model number ends with N/A?

Depends how you define Country Code I guess

In Settings, General, About, tap on that model number so it shows the 5 digit model number starting A

Then look here Identify your iPhone model – Apple Support

CN/A is Slovakia.

DN/A is Austria, Germany, or the Netherlands.

FN/A is Belgium.

HN/A is India.

IN/A is India.

KN/A is Denmark or Norway.

N/A in a eight-digit code is the Netherlands.

QN/A is Denmark, Iceland, Norway, or Sweden.

VN/A is Vietnam.

Neither the A number nor the article LD150 linked to can be used to identify the country of sale for most units.

(259549)

What you're referring to is a code that refers to internal hardware components(not just ones restricted through firmware or carrier locks), not a country code; most countries have units with the same A number, so it cannot be reliably used to determine which one it was intended for.

(259584)

AFAIK those model codes are only published in apple fan wiki sites but if you have the KB site with them I'll add it to my notes.

Most problems occur with people in one continent missing features because the phones were intended for another continent, hence the 4 major variations of Annn models.

It does not bother me if my phone was intended for Britain, France or Germany so long as it wasn't intended for China or the Middle East. The intended minor country of sale at that granularity is hardly relevant. It should not matter to the OP.

That was my point.

Is there a letter before the N?

KN - Denmark, Norway

QN - Denmark, Norway, Sweden, Iceland

HN - India