It's no secret, but it's only the rear shock that's different on the 2G. Some magazine tests erroneously claimed the naked bike had a stiffer front. The front seems stiffer because you don't have 15 lbs of fairing hanging over the front wheel, but a search of the parts catalog shows they both have the same part numbers for everything related to the forks. Suspension on the 1G naked & S is the same. I don't know about the 3G models.
The difference doesn't get talked about much because it isn't great enough for most S model owners to bother with trying to track one down, especially when Hayabusa shocks are so easy & cheap to come by and they are much better and more adjustable.
Ever ride a 2nd gen N model? All I know is mine is very stiff, front especially when set on the 3rd position as it comes from the factory. I'm 250# and have ridden 2 up with a 300 pounder on the back..lol, on the middle settings no less. If it were a longer ride, I probably would have upped the rear settings, but not the front.
The article I read, the writer had Suzuki reps available to ask that question. The factory guys affirmed BOTH front and back were firmer on the N model. I'd suspect there aren't enough naked models sold, and not many asking to purchase factory replacement springs to carry 2 different part numbers to make it worth their while to do so. Chances are anyone needing to replace either would buy aftermarket, I bet suzuki's aren't cheap.