So which one is UNF/NF? NC12, NC14, NF20?

Some of the imperial thread sizes/pitches are a little confusing.

NC & UNC are both the same

NF & UNF are the same

The threads of the seatbelt fixings are most commonly called 7/16 UNF & has 20 threads per inch ( Also known as 7/16 NF20 ;) )

Hope that helps :)