Use the Allen wrench to get the 5x12mm bolt in place in the centre hole of the tensioner housing. Fasten it to The end of the pen rail, so the slots in the housing are on the side of the rail, and the nubs slide into the extrusion on the top/bottom.
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https://cdn.masto.host/mastodonart/media_attachments/files/112/615/828/580/267/979/original/03c8bfea3109416b.jpeg