My brother just posted this on the PC-Engine forums, but I thought it was worth mentioning here as well :)
I purchased a massive lot of prototype materials from Tengen/Atari, one of which is a Turbografx-16 copy of the unreleased title "Marble Madness." I bought the lot untested, and in many instances unlabeled (as was the case with the MM prototype). I knew the game was a TG-16/PC Engine prototype from Tengen though, so I was willing to take a shot. I figured I'd end up with a prototype copy of KLAX at the very least. The lot got in today, and immediately fired up the card to find it to be Marble Madness.
I've heard stories of other copies existing, but through all my searching I haven't been able to find concrete evidence to prove it. Was there a copy for sale on Yahoo Japan? Does anyone know any collectors that own a copy? The title screen on mine is slightly different than that of the scans that are floating around, so I'm sure there were other proto cards out there at one point. The game is fully playable to the end, but I don't know how complete it is exactly.
I will be updating with pics and screen shots shortly. If you have an idea of who I am, you can trust that this is not some lame hoax. For the time being though, I'm just trying to find as much info as possible on the game.