BHP 21 I have, I got to extend the steerer tube cut too short on my trekking bike.

the long bolt is internally threaded. you tighten the steerer wedge with a 6mm allen
in it.

then you just use the short bolt thru the top cap for the headset pre load..

I chose to buy a longer common 6mm cap screw to get enough threads engaged
in the threads cut inside the wedge pulling bolt..

I think the longer bolt was custom manufactured (so as to have enough metal at the top for the internal threading
and the hex socket to be combined) .
Made just to be part of this component,

have you stripped out the threads cut inside the hex socket in that bolt?