I don't like to disagree with my lovely girlfriend Bart but the H deck is actually the very first Snap Fast model. It was introduced in 1971.
The biggest difference between your J deck and the N deck is that the N deck has a removable discharge chute to allow the boot for the Hydra-Bagger or Hydra-Vac to bolt on. The chute on your J-deck is welded on. The chute assembly for the N-deck often gets tucked away in the garage somewhere and is not retrieved when the tractor gets sold years later. If you have any intention of adding either the Bagger or the Vac to your tractor, then the N deck is for you. A new discharge chute can be fabricated for now.