The special bore makes it suitable for the extra wide (max. 3.0 mm wide) aero spokes.

Specifications of the rear hub:
Brand: Shimano
Model: FH-RS770 Ultegra
Color: Silver
Number of spoke holes: 32GTS
Type of brake: Disc (Centerlock)
Number of gears: HG Body 36.85 MM (HG 11) = suitable for all cassettes (possibly with the use of spacer(s))
Type of axle: Steekas Ø 12 MM
Axle included: No
Type of bearings: Ball bearings
Average weight (GR): 293
Intended use: Race

Data for spoke length calculations using the Sapim spoke size calculator:
Total length of the hub (MM): 142
Non-gear side (MM): 36
Gear side (MM): 50
Hub diameter non-gear side (MM): 44
Hub diameter gear side (MM): 44

Link to spoke calculator: www.sapim.be/spoke-calculator