The only way to eliminate the typically unused space between the cassette and the hub flange was to combine the freehub and the cassette into one piece. The 21t gear of the cassette is aluminum and contains the freehub, while the last 6 cogs machined from a single steel piece.
EXA+ Alloy and Heat-Treated Cromoly Steel
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Guaranteed to be free from defects in materials or workmanship for 5 years after original purchase