Originally Posted by
Bikenut61
I have the WH-6600 pre-built wheel.
You are limited to the freehub body on it and the 10-speed cassettes that were designed to fit those taller splines.
That wheel's hub, and it's Dura Ace contemporary hub and prebuiit wheel (HB-7800 and WH-7800), used a different freehub and hub bearing design than any other Shimano freehub before or since. The drive side bearings were inboard of the freehub body, not at the outer end of the body, and these bodies are unique to these hubs.
The taller splines were used to let Shimano provide a lighter aluminum freehub body without the cogs gouging them. It was considered a failed experiment and Shimano quickly reverted to the outboard driveside bearing and steel or Ti freehub bodies with the older short splines.