Even the most die-hard Nintendo fanboys could be forgiven for not knowing about the 64DD. The 64DD was a magnetic drive that snapped onto the bottom of the Nintendo 64, slated to run bigger, rewritable games and give the massively popular console internet connectivity. After four years of development delays the 64DD finally launched in Japan in 1999. Only 10 games ever came out for it -- fewer than the equally embarrassing Virtual Boy -- and Nintendo eventually scrapped the Western release.
Since flopping on the consumer market the 64DD has become highly valued among collectors, and over the years some rare developer units and prototype discs have surfaced. They can sell for thousands of dollars each. YouTuber MetalJesusRocks thought he had both, but it turned out he had something much more precious.
The working 64DD unit MJR got through Craigslist had one very strange feature: The boot-up screen was in English. Why would such a thing exist if it was never released on Western soil? His first theory was that it could have been a developer unit, but Mark DeLoura, a former lead engineer at Nintendo, helped disprove that. This was an honest-to-goodness retail model that had gone through quality assurance (hence the unusual "lot check" sticker on the front).
To make matters stranger, the copyright date on the bottom of the unit is 1996, 1997 -- a full two years before the add-on appeared in Japan. Maybe this was part of an initial run of early models before Nintendo decided to scrap the Western release? Whatever the reason, the existence of an English-language retail model of this rare and largely-forgotten peripheral was long considered a myth according to MJR.
Though his 64DD did come with one of those rare blue prototype discs, he's been unable to get the system to read the disc's contents. So if you have any information on how to see what's on it, send him a tweet.