The highest recorded SN for a 2nd gen NF is 7288NF. All serial numbers above that and any that start with a zero are 3rd generation NF's.
If you have one slightly above 7288NF like 7290NF, I guess one could argue that it 'might be' a 2nd gen.
The New Frontier situation can be confusing to many, understandably so.
The first production New Frontiers were made in 2nd Gen SAA production. Some could assume this is a '1st Generation NF'
The next run was during the earlier 3rd, which I have seen some refer to as '2nd Generation New Frontier'
Now we have the 3rd production run of New Frontiers which is being made during 3rd Gen SAA production. I guess things finally match up haha.