I managed to find my post where I documented some of my findings re: Shimano freehubs. You can read it here: http://www.bikeforums.net/showthread...ht=shimano+hub

I have more detailed notes somewhere but who knows how long it will take me to find them. Regardless, I note in that thread that the freehub spacer was identical between an RSX hub and an Ultegra 6500 hub. I swapped the freehub body on that RSX hub for one from a Tiagra hub so I'm certain that I have an extra freehub spacer in my parts bin. If you can't track one down, let me know. Should only cost a postage stamp to mail to you.