Right, so. If you’re like me you finished the quest “Aquarius” before starting the quest for the “Queen of Sheba’s Sword”. Sadly, it is bugged and you can’t fix it; however, while doing some treasure hunting…I found it! It is in the grave from Treasure Map 25, and usually it would be a “Razor” but for me it appears to be Queen of Sheba’s sword! Hopefully this helps anyone else.
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Or you can find the sixth piece. 
There is another third piece in Rattay? I have one from pickman and the merchant from his second floor.
In the blacksmith’s house (The Blacksmith near the Upper Bailey / Upper Gate) in the storage room it is stored on a shelf.