The sixth and final Bureau can be found in Wincestre, just south of St Peter's Church. Inside you'll find the last Codex Page and Suttungr's Claw, a dagger.
Drop down into the water and dive along the underwater passage to get on the other side of the barred door. From there its simply a matter of bringing an oil jar over the water and blowing up the rubble blocking the entrance to the Bureau.
I've included in the video the conversation with Reda that happens once all six pages are collected. You're able to read an interesting letter from Bayek of AC: Origins
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