BKB: Black King Bar.
It's a combination item from Blue-Black Stone and Monkey King Bar (MKB)
S&Y: Sange & Yasha.
These are double-katanas (Or supposedly two seperate katanas that work perfectly with one another). This is also a combination item from two katanas: Sange, and Yasha. They're also in that link that is above.
Yet another combination item. It's a mechgun with (I think) +200 atp. But in order to get it you need to combine Handgun: Guld and Handgun: Milla. Both of which Whitill can find (good luck) and are both extremely rare.
Sealed J-Sword: An item that is so far only reported for Skyly, I don't know much about it. You can check it out:
Also it supposedly can unseal, which nobody has yet proven. That would make it the Tsumikiri J-Sword
All of these items are considered as uber rares, but mainly they are combination items. The J-sword is not to my knowledge. But you never know since changing it to the Tsumikiri J-Sword has not yet been proven.