I've seen some discussion about a possibly fake Jap Heavy Sixer on this thread:
And then this old topic was rediscovered, but the images are gone.
I was able to acquire a Japanese Heavy Sixer in Brazil, it's in my hands right now, and, as far as I can tell you, it's legit. I have no second guesses about it.
From what I've learned so far, it seems Japanese company Epoch Co. has imported some Heavy Sixers from Sunnyvale before the Atari 2800 was officially released in Japan. It's credible, cause the same thing happened in Brazil. Before Brazilian company Polyvox release Atari licensed consoles, some Light Sixers were imported and have experienced some rudimentary adaptations such as adhesives and partial translations (I have two units of those rare consoles).
In the case of the Jap console, I was able to acquire a CIB one, and so we can understand what Epoch did at the time. The adhesives match (they are on the box, on the console, etc), there is a matching new serial number on external and inside box, it doesn't match the console serial number, I figure it's an Epoch internal number.
Also curious is the games adaptation. Space Invaders have a rude sticker attached, I could say they would ruin the box, however, nowadays, it's a proof of origin, combined with the Jap manual. Combat is interesting, there's no sticker attached, and the original US manuals are preserved, possibly because the Jap version wouldn't fit in the gatefold pocket. So the new manual was inserted into the box (below the cart). Of course Jap silver games are much more beautiful, IMHO, however, those "first imported games", for me now, they're maybe even more valuable.
Power adaptor, RF box, console manual, everything shows this is authentic.
Hope you like it! Enjoy.