Strange single adjustable aluminum bodied and Ultra struts have a hex shaped adjustment on the top of the shaft. Turning the hex clockwise will decrease extension (rebound) and counter-clockwise will increase extension (rebound) resistance. The strut has 2 ½ to 3 turns from stop to stop. The adjustment should be done very gently and slowly, since the hex is connected to a thin rod, which fits inside the gun-drilled strut shaft to connect and adjust the rebound at the bottom of the strut.
Another little tip, never leave the adjuster all the way loose or tight, The next time you try to adjust it if you forget which way to go and turn it against the stop, you can easily break the adjuster.