OK, That one is fairly simple internally. But to get it apart you have to remove two split spring rings, one at the front and one at the rear. Then it splits in half long ways.
The fish eye lens sits over a wavy spring. It should just push in and rotate a bit then come out. But they don't always do that. Sometimes they just get stuck then you have to take the housing apart and then remove it. I suspect that's why I've seen so many with out the lens.
Joe