This Blocklock Crown Race enables the secure mounting of forks with a 1.5“ steerer tube diameter on frames with a ZS66 Blocklock headset.
Particularly practical: this headset bottom allows you to easily switch between different steerer tube standards – for example, from an original 1.8” fork to a fork with a 1.5" steerer tube. The steering angle possible with this fork cone is 150°. 
(So replace an oversized steer tube 1.8" fork, with a standard 1.5" fork.)