They're legit. The first generation of North American carts were designed like that. (I had a first-gen US SNES, and that is exactly what the bundled Super Mario World looked like.)
They were designed this way so that the cartridge couldn't be removed while the system was powered on-- the power switch was connected to a plastic bit that slid into the notch when the switch was pushed in.
I can't recall why they changed it or when... something to do with cartridges being too easy to get stuck, I think?
Edit: Looking around other forums... there's no official explanation, but the consensus seems to be that yeah, the design was changed because people tried to yank the cartridge out without the power switch all the way in the 'off' position and ended up breaking their systems that way. -_- Guess even Nintendium has limits on its indestructibility...