Mathieu, I can also confirm that the cartridge is consistent with other Data East Protos which I have seen over the years. Magical Drop 3 & Street Slam immediately come to mind. They all have the same characteristics. Clear cart shells, all rom positions are socketed, small square shaped Texas Instruments eprom window stickers with black writing, cut out vents & sections of the cart shell, data East logo on outer seal stickers, and label maker sticker labels. All of those things are very commonly found with their test carts.
One other thing though, for the historical record, this cart is not a new discovery. It was found in 2004, in Japan, about a year after my cart surfaced in 2003. In fact, I have pics of that cart (and other data east Proto carts) from that time period that were sent to me. I'm not at home at the moment, but I can search my old files & post the pics here later this weekend.