I have a few genuine Japan batteries from when I owned DX90 (went a bit nuts and hunted for them everywhere), I ended up hitting gold with the official Samsung accessory packs, obviously I had to open the packs to find out whether I had scored a Japan battery, from 6 tries I luckily got 3 Japan batteries, 1 Korea, 1 China, 1 Vietnam, at that point I already had 1 Japan battery I was using which I got from a S3 phone so that's why I went on the hunt for more Japan batteries, because I knew they sounded awesome. I never thought I would sell my DX90 otherwise I would have kept the packaging, I have the chargers though, I have tested them all and I
guarantee they are genuine, there was never any fake accessory packs firstly, but more importantly I have directly compared these official batteries to fake samsung batteries I bought as single units from amazon/ebay, the fakes even have made in Japan written on them and look
identical to the genuine batteries from these official accessory packs! But they sound similar to the stock battery, so that means the only way to make sure you got genuine is if they came in proper samsung packaging. Also genuine single unit samsung batteries all come in a proper box, if you received them in a plastic sleeve like the ones from ebay then they are very likely fake like some of the ones I tested.
I have for sale 3 new genuine Japan batteries for $45 each, will come with the charger, 1 new genuine Korea battery $35, and 1 Japan battery that I used in my own DX90 (the un-sleeved one) for $25. I kept all batteries at 50% charge for optimal health. And btw I'm not making money of these, selling for cheaper than the accessory packs cost me, and I'm losing almost double my money when you calculate the costs it took to obtain these out of the fakes and other genuine units I went through... But I have no use for these anymore, if you have disposable income and want a boost in sound quality these Japan batteries are the best.
The Japan batteries obviously sound the best, the Korea battery is second, and the genuine China and Vietnam batteries sound better than the stock dx90 battery, the fake samsung batteries are similar to the stock battery in quality.