On our Duo, I did not notice any aluminum in the steer tube. The setup uses an expanding type fastener that wedges to the steer tube ID via an internal wrenching bolt. The stem cap is special having internal female threads that engage male threads on the wedge fastener.

If our fork does have aluminum in the steer tube, I must have overlooked it or it's bladder molded internally and compression molded externally. It does if I recall correctly have a lower crown race aluminum seat securely mounted to the lower portion of the steer tube.

Regardless, I need to give it an inspection, in good light.

PK