As our friend CloudGamerX here has shown us, friends, is that you never know when a prototype could show up. I was thinking, should we attempt to scour eBay for various label-less cartridges for any system and hope that there is a prototype among them? It's taking a shot in the dark, but it could pull up some results. As we know, EEPROMs need not always be exposed for there to be a development board inside. There have even been instances of color labels being printed for review copies as is to be seen with Drac's Night Out on NES. Then, you may occasionally find one bearing a black and white label with the title image, such as with Zero Tolerance for Sega Genesis. Should we go forward with this by posting links in this thread to give people the opporunity to investigate them?

A couple other copies of Xexyz like that have been found.

You have to be careful. I only noticed it was a proto based off comparing it to other Taito protos and official retail PCB scans. For example, being a late release Taito game, it should not have all the edge connector pins, but the pictures showed the cart did.

What the heck? I had an in-depth posts about labels and soldered versus sockets in my post above and it's all just gone.  

I recall pointing out that socketed ROMs are more likely to be different from retail because soldered in ROMs are necessary for review cartridges to fit properly in a normal NES (for 3rd-party review of a final or near-final version). My point with Xexyz protos was that they sometimes do have graphic labels.