Originally Posted by JulesCW

Are both sides of the spacer allen-head fitted (likely 5 mm)? If so, try inserting an allen from each side and unscrewing it that way -- be careful to insert the allen keys far enough to engage but not far enough to protrude past that middle washer. I've done that on a White Industries hub as well as on a Phil Wood -- it would not surprise me if this worked that way, too -- then you put the Arai where the washer was -- in my case both hub spacers were designed just for that use.