75N and 75R were the designations before NW expanded the F cabins to 22 seats. 75N became 756, and 75R became 755. For a few months, all four types were flying, but the conversion was pretty quick, and the 14 seat cabins should be long gone (good riddance!).

The difference between the types is the door configuration--the 756 has 3 doors and 2 overwing exits on each side, and the 755 has 4 doors on each side. so the exit rows in coach are different, but in F, they're interchangeable.

i've run into the 75N designation a couple of times since the switch, but haven't seen the 14-seat cabin on any of those flights. You should just get the 756.

so yes, in the early days of the CO alliance, it was often easier to upgrade on CO. :-)