Bronica backs are confusing since everything came in different versions and i think most everything works in every version, so it's quite possible to get the back, slide and insert all mixed up and then sold to you.
As far as i can tell, there's the single latch backs, then the double latch backs. There's earlier inserts, with a kind of gear thing to get the film loaded, and later inserts where they've put on a little lever like on 35mm camera rewind cranks - i think the later one are supposed to have better film flatness, but i can't tell the difference. The dark slides have either a black or a gray handle, the gray ones are the ones that lock. I don't have any gray handle ones so i can't tell you exactly how they work, but i think it's only with the later backs.
I have the 135W back, and it's pretty great - 22 shots on a roll, panoramic pictures, being able to use any ol' 35mm film. I think the 135N back would make more sense if there were lenses past 40mm. I guess if you've got a macro setup or a big telephoto it would be nice though.