An annonymous tip from one of our Forum readers clued me in on a new design patent awarded to Nintendo for what appears to be a cartridge of some kind for the N64. The Design Patent number is D427,245. I don't know what this is, at first glance it appears that it could be some sort of a lock-on cartridge, but upon further examination, this no longer seems to be possible.

Note the bottom portion of the design is exactly like a N64 cartridge, however, the unit becomes larger near the top with what looks like 2 small, thin slots. I don't know what could plug in here. Design documents do not have to be exact in order to be appoved, so the illustration could be misleading. It could be used to plug in GB or possibly GBA cartridges, or it could be used for some sort of other type of memory card. I'm willing to bet that this is some sort of development tool, though.