That doesn't look like a factory roof rack, for starters. I believe they only came on the crew cab, but I do remember somebody taking a crew cab rack and cutting it down to fit a king cab. I just don't remember if it was a 1st or 2nd Gen Frontier?
I'm nearly certain only crew cabs ever got a factory rack. If there was some configuration that put a rack on the cab of the king or standard cab I've never heard of it or seen it.
I remember seeing at least 1 person on this forum shorten a crew cab rack to put on a king cab. I believe it was fusionfronty99 or something like that. There may have been others as well.
ETA: I just looked at that thread posted above. In there I posted a link to the members truck I reference above. However, thanks to photobucket being dildos, all of the photos are messed up. You may be able to go to His album on photobucket and view them if they aren't set to private.
edit 2: If you want to see the picture that gets replaced by photobuckets "third party hosting" bs you can right click it and select "open image in new tab" if using FireFox. It will show you the picture on photobucket's website. Or right click and copy the link then paste it in a new window if youre using a different browser and dont have that option. Here is one of the pictures with his modified rack: http://i33.photobucket.com/albums/d55/JBurtonator02/P8020634.jpg