My take on it, they fit 1,2 and 3 with the existing spacers for the models in question... so Gen 1 and 2 will fit straight on as long as you fit the existing carrier, spacers etc as per what was removed. Makes sense to me.
I have a gen 3 set of wheels on my gen 2. They came with sprockets and spacers. It was a straight swap of whole unit with sprocket carrier, spacers etc. I did check spacers carefullly against mine & they were identical. I think the gen 1 spacers may be different.