Kasm is a 3D printed slimline return top designed for modern responsive (0A) play.
Bearing: Size B
Response: YoYoFactory small bearing pads
It uses a small bearing, but also comes with "bearing locks" (red ring in the last picture). Putting one of those rings into the bearing seat will cause the bearing to lock up, using the outer race of the bearing itself as a fixed axle.
Be sure to stock up on some extra bearings and parts.
Kasm shares and improves some design features on Butterfly Rims and Crystal Rims for Mod44. Checkout this post and this post on how they are related.