Manual says 1/2unf X 5”. 60 ft lbs torque
That will be the bolt for the through the centre of the tube mount.
The lower mounting point will be much shorter.
1/2" UNF. Approx 62mm long.
well spotted - just checked the amendments and its 1/2”unf x 2 1/2” for the under tube mount. Uses plan washer either side of the damper and lock washer under the bolt head.
If an old dedion tube also check the integrity of the lower mount particularly where its welded to the tube - early versions can peel out. Later versions have a thicker walled tube and extra buttress plate welded on.
Looks as if this one is solved.
In case anyone hasn't found these yet: Bolt List and Fastener Packs.