This is a Ergo Grip Vertical Foregrip, model 4231-BK.
ERGO's KeyMod Mini Max Vertical Forward Grip (VFG) is the most versatile grip available. It can serve as a traditional VFG" but is small enough to be used like an angled forward grip or a forward hand stop. It is a rugged" multipurpose KeyMod accessory built for hard use and solid performance. Attaches directly to KeyMod slots to reduce bulk and weight and its aggressive machined surface provides for a secure support or grip.
Features:
Mounts flush to KeyMod slots
Black anodized 6061-T6 aluminum
2.75 ounces and 2.5″ in length
Fits KeyMod slots only