This is a pair of Prometheus Flat Hop nubs.(4560329179178)
The "Flat Hop" is a very popular modification due to its simplicity as a moderate accuracy upgrade.
Traditionally, the Hop-Up arm and bucking need to be modified to create a Flat Hop. However, Prometheus has taken things a step further by creating their own drop-in Flat Hop system. This ensures an easy installation and maximum performance from the Hop-Up parts. These Nubs are meant to be used in conjunction with a Prometheus Straight Bucking.
This set contains a soft and hard flat type Nub.