I've never heard what the (N) stands for, but I do know that it's only on the latest boxes.
I have 35-90 and 35-90B from the earliest to the latest box styles. The newest stuff all has an (N). XLI 35-90B(N) and UD 35-90(N). The only obvious difference is the reel style.
They may have changed something about the formula, which they have a few times over the years as have other tape manufacturers, but I don't know. Or it could be as simple as a different reel (New reel?) since the newest ones are different than the classic reel style we were accustomed to. The old website gives no explaination.
I would kinda like to know... ‘might give them a call.
Other than the box and reel styles the tapes look, smell and behave the same. 30 years of consitancy… the manufacturing facility must be built in a pyramid or something.