I actually had this one my watch list, but meant to do a bit of research into this. Aleckermit just shot me an email however, with a bit more info and a heads up that all the copies the seller had have been sold.
It seems this may have been a replacement for a recall Nintendo had on the origional cart, due to an in-game glitch.
A couple sources hint that this cart was offered as a fixed replacement for those who sent in their black cart back to Nintendo. Seeing as the game probably wasn’t a huge success, followed by the fact that the black cart glitch only affected co-op, these replacements were likely in very low demand. I can’t find any sources of anyone that’s seen/owned the grey cart before now. There’s a very slim chance that it was never released, but that’s just speculation for now until people come forward with NTSC greys that weren’t purchased from the recent seller. In any case it’s a very rare cart variant.